Okay I was able to test this. Note that my internet connection isn't 1+1 yet because I'm not yet in my new house. But I tested this one using my 1gbps M1 and via USB mobile tethering. Because anyway the principle should be the same.
Pre-steps is two check the ISP IP vs the mobile data IP. I checked it via https:// whatismyipaddress . com/
By right, the IP addresses should be different. This is very important to check whether I was on the right track later in the testing.
So going to the DUAL WAN page settings in my Asus Router (rt-ax88u), I set my USB Tether as Primary WAN, my M1 as secondary WAN.
i checked the IP of my pc and my other Mobile device via (S24 Ultra) and set its ip in the routing rules section. Both are connected to the Primary WAN. Reason being is that my usb tether is locked to 4g so i can check the speed as well to determine whether it is working..
I apply and tested. So far my pc and s24 ultra when i check whatismyipaddress, both are showing the ip address of the usb tether (Primary WAN) Speedtest also is around 100+ Mbps which means it's using USB Tether (Primary WAN).
I connected via wifi an ip13pm and s8 tab, checking whatismyipaddress shows the IP address of my ISP (M1 1gbps) and speedtest is above 500+.
I'm quite happy with this testing because it appears to me that using routing rules should be quite simple. This is just my first testing though. I would like to see still once I have my 1+1 but i guess this should work.
My use case is that 2 of my pc connected via LAN is in 1 line, then all other connected devices should be in another line. I need to ensure that the IP of my PC should not change. This is still something I'm exploring coz...i'm noob.

Hope the above helps and the experts can add on in my experiment.
PS. As I type this in my PC, this is still connected to the USB Tether primary WAN.
