Add a repeater to the mesh

Repeaters act like the backbone to the mesh, they take traffic and repeat it, hence the name, they keep doing this until the traffic gets to its destination, and the more they are over an area the bigger the mesh can be, if you want to add one, here is how. (specific instructions for RAK Wisblock (RAK4631 and RAK19003/RAK19007) are here).

My recommendation for repeater hardware is the RAK Wisblock (RAK4631 on the RAK19003 or 19007 baseboards) due to its extremely low drain on batteries, the option to directly connect a solar panel and the ability to update the firmware OTA via Bluetooth

1. Purchase LoRa hardware, it doesn't need Bluetooth as we don't use Bluetooth for repeater access, this is done over the mesh itself or via USB at initial setup, hardware that can be used for this includes the Raspberry Pi Pico and the Waveshare SX1262 LoRa module (make sure this is the HF 868MHz version, there is a 433MHz version as well which is not compatible with the mesh), or the RAKWireless Wisblock series, usually a RAK4631 and a RAK19003 or RAK19007 baseboard, both of these have an option for a solar panel input, some of the companion radio devices can also be reconfigured as repeaters, the Heltec V3 for instance.
2. Purchase a suitable outdoor antenna, these must be for 868MHz, Paradar and McGill Microwave sell antennas for this band with varying amounts of gain on them, if you're not familiar with RF and gain then in layman's terms it just increases the signal, if the repeater itself will be some way from the antenna then you must use appropriate coaxial cable, Paradar and McGill Microwave also sell this, ensure the coax is ONLY as long as you need it to be, no longer, due to the very low power and signal strengths involved we need to keep losses between antenna and radio to a minimum
3. Optionally you may purchase a solar panel to charge the batteries if the repeater will be away from mains electricity but you will need batteries of a suitable capacity to ensure the repeater stays on air for as long as possible, both these items are obtainable from Amazon
4. Once you have your LoRa hardware you will need to flash some firmware to it, this is available from the MeshCore website, except this time you will select the repeater firmware, you should make sure an antenna is connected while doing this as when the board reboots it will send an advert out, keep this connected to your computer for the moment as you will need to make some configuration changes, such as a name for your repeater, its location on a map, the admin password, how often it adverts both flood and zero-hop and also sync the clock if needed (note you may need to do this again if the time is not kept between you disconnecting the USB cable from the device and connecting a battery and other parts later on)
5. With the flash process complete fit the hardware into a suitable waterproof enclosure, the RAK hardware can be used with the zerofox3D Hermes case, this comes in a choice of N-type or SMA antenna connections (most antennas for outdoor use will be N-type) which can simply screw on (with an adaptor if this is needed) to the case, zerofox3D also provide this case with a 4x 18650 battery holder and wiring for a suitable 5-volt solar panel, plus there is an instruction video on YouTube on how to assemble this case properly, other hardware will require a suitable waterproof enclosure for the task and would require holes drilling for antenna connectors and solar panel wiring (if you use this) to pass through which have to be made watertight, though in all cases waterproofing is less critical if you put a repeater into a loft though I don't recommend this as it could attenuate the signal
6. Once the repeater is in its enclosure it is ready to be installed, try and get it as high as you can, at 868MHz this is extremely important, ensure the antenna connections are tight but not over tight, if you are using a RAK device with the zerofox3D Hermes case the antenna can simply screw in as noted, make sure before this goes up however that the connector is waterproof by using self-amalgamating tape, this is readily available and will work to help keep water out of the antenna connectors and coaxial cable where you are using this, position the solar panel if you are using one in such a way that it will receive the most sun on it all year round, then switch on the repeater.
7. You should be able to access the repeater using a MeshCore dedicated device or the app and companion radio using the password you set for it, there is a 15 second wait for this but this can be lifted if you donate £4.99 to the MeshCore project in--app, once into the admin panel you can make changes here, be aware some data won't be shown to save bandwidth and you can see this by pressing the refresh button, at this point I recommend syncing the clock again just in case the device has lost the time, the repeater can also be made to send a flood advert from this control panel and you can see any repeaters that are zero hops away using the 'Neighbours' function, if you have yet to set the location you can do this from the app in the same way you would set your location for your companion device, just make sure to read this from the repeater first, there's also a command line function and a list of common commands can be accessed through a menu in the app so you know what command does what.
8. At this stage you should have your repeater up and running, you can optionally allow guests to view data from the repeater, this doesn't need a password by default, the data available is the battery voltage, clock, uptime, total airtime (this won't be the same as the uptime), data relating to signal strength and signal-to-noise ratio, and data relating to packets that have been send and received by the repeater since startup.

So to help the mesh grow all you need to do is add a repeater, preferably mounted as high as possible