Multi-wan failover without changing IP?

Hi, I'm in need of some help. I don't know much about networks, so please I would appreciate simple explanations :(

I play an MMO, and recently I've been having network issues. So I did some research and asked around and was told to get a multi-wan router and setup failover connection, using phone tethering as a backup. So I got the Omada ER605, but I don't know how to set it up properly.

I managed to set up the failover, but there are 2 issues:

1. The takeover takes over 10s, making it worse than the disconnects I have been getting. 2. The IP changes, and I need to relog into the game at which point I'm already dead.

Any way to get over these two issues?

Tech specs: \- I use Linux Mint \- Router firmware at v2.20