You need a router with multiple wan fail over support. You could build your own with PFSense or Opensense etc or buy one the supports out of the box.
Best one to buy for your situation IMO is a Pepwave Balance 20x [1], they are rock solid and specialize in multi-wan with a bunch of options from simple fail over to bonding them all together with vpn tech.
With the Pepwave you could put the 4g sim directly in its modem then get 2 wan licenses and plug the two DSL modems into them and set priorities as needed. Ideally the DSL modems would be in passthrough so you don't double nat but should work either way.
I use one of their routers in my RV and it seamlessly and reliably switches between Starlink via ethernet, 5g cellular and WiFi WAN.
Comments
You need a router with multiple wan fail over support. You could build your own with PFSense or Opensense etc or buy one the supports out of the box.
Best one to buy for your situation IMO is a Pepwave Balance 20x [1], they are rock solid and specialize in multi-wan with a bunch of options from simple fail over to bonding them all together with vpn tech.
With the Pepwave you could put the 4g sim directly in its modem then get 2 wan licenses and plug the two DSL modems into them and set priorities as needed. Ideally the DSL modems would be in passthrough so you don't double nat but should work either way.
I use one of their routers in my RV and it seamlessly and reliably switches between Starlink via ethernet, 5g cellular and WiFi WAN.
1. https://www.peplink.com/products/balance-20x/