Cheap router for LTE backup

Trying to add something inline between my cable modem and my wifi router (google wifi) that would detect when my internet goes down and would automatically switch to using a LTE dongle (or even 5G if those exist yet).
I guess I want at least 2 Giga ports and a USB 3 for the dongle. I don't need wifi for this application.
Hopefully I can find something in the $50 range.

A 5G cpe (search for oppo cpe t1) is about 400 bucks. It has 3 gig ports and WiFi 6 (ax) but no chance for openwrt.
In your price range you can only get a mt7621 device with 5 gig ports and USB 3 (search for dlink dir860l only B1 hw)