Oh dear, this is maddening....
I've got a Speedybee F405 wing, a Foxeer elrs receiver, and Radiomaster Boxer tx.

The tx and rx are bound. I've got a solid led on the rx. which changes colour according to packet rate. However, there's no stick input on INav, I can't set an arm switch - nothing. I've got tx to rx, and rx to tx on the rx wiring, the soldering is good, the correct uart has been assigned, and crsf has been chosen.
I do get 'rf critically' low message when clicking on the 50hz packet rate. And get get 'telemetry lost/recovered' when unplugging and replugging the rx.
Model match is off.
I haven't however flashed any rx firmware, or set a binding phrase. I DID set a binding phrase for an earlier model with the same tx, but this was with Betaflight. Could this be the problem?
I also successfully bound and flew a mini drone (mobular 7) with the Boxer, without messing around with binding phrases - it just immediately worked with Betaflight.

What more can I do?
Please, feel free to ask as many questions as you like!

This is the same for me. 

Matek F765
Radiomaster RP3 elrs RX
Radiomaster TX16S Mk II

RCX is bound. I have updated RX and TX firmware. Wiring is correct. Correct UART selected. CRSF has been selected.

Thanks.

Is model match disabled? In the ELRS lua, turn model match on briefly, then off again.

If that does not fix it, put both TX and RX into WiFi mode and show screenshots of the webui.
https://www.expresslrs.org/quick-start/w...the-web-ui

Also consider jumping on the ELRS Discord for troubleshooting: https://discord.com/invite/expresslrs