I've been quite excited to get this thing up and running since we have marginal LTE coverage here and I'm currently using an omnidirectional antenna.

When I fire up the router, it works fine for a few minutes then goes off the network. It doesn't send any more packets (the green network light still flashes). A reboot isn't enough to recover it, I need to hold down reset to get back to factory settings.

I thought this might be software related but I've just updated to stable 6.48.2 and it's still happening.

Is there anything else I can try before RMAing it?

What is the status of the LTE modem when this happens?
Is it running? "R" flag...
What does the status tab of the LTE interface indicate ?
What does the log say ?

There was nothing unusual in the logs, it appeared to hang after a few minutes.

I think I've traced this to the PoE injector that it was supplied with. It runs fine connected to a switch

On a POE switch you mean.. i guess...