My Homelab currently consists of 3 Mini PC’s and will eventually be put in a 10" rack

They are all just plugged into the router my ISP provided, I’d like to get a new router that runs open-source software and create a new network from it. I have no idea where to begin.

What hardware would you recommend?

Bonus: If possible I’d like to in the future attach a sim card to my network as a backup for the occasion that the ISP connection drops. (just a nice to have)