Setup 2 ethernet Interfaces on Ubuntu Server 20.04 -- 2

Hi, i would need help to setup 2 LAN interfaces on my raspberry pi. I have connected 2 4G-Routers behind them. So what i exactly need is this workflow: eth0 interface is default for all in/out traffic eth1 is also connected but in standby so there is no traffic I want to switch the traffic from eth0 to eth1, without any interruption. After that switching may i reboot eth0, and later i switch back the traffic from eth1 to eth0. What i tried until now is work with 'route add default gw eth1', and when i want switch back, 'route delete default gw eth1'. The problem is when i switch to eth1 and reboot then eth0 manually i cause no internet connection. Im searching for an command line where i can switch between these 2 ethernet interfaces, and be able to reboot disconnect or power off the other ethernet without effection the present working one, and the switching need be without interruption of the internet.