USB LTE modem recommendation for backup connection

Hello guys,

What would modem you recommend for LTE backup connection (USA, AT&T or T-Mobile)?

I have WNDR3700v4 with OpenWRT router dedicated for IOT network and would like to setup backup connection in case main goes down.

I do not expect huge traffic or long outages so high performance is not priority.

Thanks

an old phone

How it can be connected to router as USB modeb?