DSL main, cheap little LTE modem via USB as a fallback. Both are connected to my OPNsense as a gateway group. Failover happens after 5s of full packet loss (and a bunch of less aggressive failover conditions, latency etc.). That of course changes my public IPv4 address, so yes this drops existing connections. Not a big deal for most stuff, Netflix reconnects quickly enough that this isn't even noticeable. For the stuff where the connection can't drop like that, I run a VPN tunnel on each of the two uplinks to a small relay box with a static IP sitting in a datacenter. When DSL fails, the traffic is routed through the other VPN link but comes out of the relay box with the same public IP.