Sounds like you need a dual WAN router, and you’ll need to set your current two ISP modems so they are not assigning IP addresses. There are some consumer dual WAN routers you can get, I think Asus has some models. You can set them to use one ISP as the primary, and when that drops out, the router automatically uses the secondary. When the primary is back online, the router automatically changes back. You’ll get perhaps a few seconds of dropout but it shouldn’t need any user intervention.

Can’t speak for your provider, but if your router only has 1 GbE ethernet ports, this isn’t possible. You will need at least 2.5GbE ports to get this speed.