I'd envisage we use the LoRaWAN protocol. The end nodes are only going to transmit small packets containing values from the sensors. 

Transceivers

So we've got each detector (the end node) with an RFM9x LoRa packet radio at either 433 MHz or 868/915MHz. Important to note legality of using these frequencies in different countries! 

Specs for RFM9x from AdaFruit:

• SX1276 LoRa® based module with SPI interface
• +5 to +20 dBm up to 100 mW Power Output Capability (power output selectable in software)
• ~100mA peak during +20dBm transmit, ~30mA during active radio listening.
• The RFM9x radios have a range of approx. 2 km line of sight with tuned uni-directional antennas. Depending on obstructions, frequency, antenna and power output, you will get lower ranges - especially if you are not line of sight.

e.g. RFM95w 868 or 915 MHz  $19.95 + antenna connector SMA SMT etc. + actual antenna 

Receivers 

Then we've got a gateway to receive the packets from the end nodes. Something like the concentrator module iC880A 868MHz (https://www.thethingsnetwork.org/marketplace/product/lorawan-concentrator-868mhz) which is able to receive 8 packets simultaneously

[more details https://webshop.ideetron.nl/Files/3/1000/1211/Attachments/Product/g8VXtyg094107xP4XTa4Ot095y24E31j.pdf] alongside a host MCU. This could be a Raspberry Pi. Or a PC! And an antenna.

Topology

Star-of-Stars network topology for LoRaWAN https://www.researchgate.net/figure/The-LoRaWAN-network-topology_fig1_340100897

Well there isn't going to much fun regarding line of sight without obstructions in a forest between the end nodes and the gateways! Do we have the gateways mounted on tops of trees with directional antenna? So the gateways are just really packet-retransmitters now? This is going to take a bit of investigation!

General layout: gateways communicating with servers via 3G/4G

General layout: gateways are now also packet forwarders, with repeaters between them and servers. 

So I found some ideas for using repeaters and making the gateway software do packet forwarding!

https://github.com/Lora-net/packet_forwarder/issues/80

https://github.com/Lora-net/picoGW_packet_forwarder

And idea for the repeaters is just this Challenger RP2040 LoRa (868MHz). It's the RPi  RP2040 MCU + the LoRa RFM95W modem. See e.g. https://www.tomshardware.com/news/raspberry-pi-pico-challenger-rp2040-lora-revealed (~£22). Antenna via U.FL connector

There's an interesting rocket project using the RFM95W modem here https://johnjonesfour.com/2020/10/03/model-rocket-telemetry-part-1/ as a breakout for the RPi Zero. Presumably would be fun with a high-altitude balloon too!

Regulatory

Frequency plans by country https://www.thethingsnetwork.org/docs/lorawan/frequencies-by-country/

Regulation overview. Take note of duty cycle and maximum emitter power! https://www.disk91.com/2017/technology/sigfox/all-what-you-need-to-know-about-regulation-on-rf-868mhz-for-lpwan/