How can I force traffic through tether instead of WAN?

So I have a GL-MT300N-V2 “mango” with just one WAN port, one LAN port, and a USB port capable of tethering to a phone. What I am trying to do is route everything through the tethered phone by default, and use the other WAN connection as a fallback.

I got it to work for about 30 seconds using a static route but then it quit for some reason, also both ISP’s I’m using are dynamic so I don’t think it would be a long term solution.

Can anyone point me in the right direction? I have 0 networking experience because internet is very scarce where I live. I apologize for my ignorance.

Hi, you can adjust the priority parameter ‘metric’ in /etc/config/mwan3

Yes I have tried setting the metric lower for the tethered interface and that does seem to do something but from what I have read online this only effects outgoing traffic. (Which seems correct because the bulk of the incoming data still tries to come through the tether.)
I have been messing around with static routes some more but it keeps switching back to the tether whenever there is some kind of hiccup with the tether. (the service is pretty unreliable). Again, I am completely new to any real networking stuff but I would greatly appreciate any kind of help. Perhaps there is some kind of script/plugin I could use? Anyway thanks for your reply lancer.

Hi, the router use MWAN3 to manage multiple WAN connections.
You can follow the link for more information. [OpenWrt Wiki] mwan3 (Multi WAN load balancing/failover)