This is due to the standard rules of IP addressing and routing. The way I deal with this is open a WiFi ssid on my modem router, and connect through that if I ever need to access it.
I’m sure there is probably a way to create a sub interface with another ip in same subnet as your router, but I’ve never gotten around to exploring that.. perhaps someone else will chime in with best practice way to configure an admin interface for modems running in bridging mode.
Edit:apologies I see you’ve actually already tried to get an interface configured within the same subnet.
In my experience, no special interfaces or routes have ever been necessary for me to access my cable modem (192.168.100.1) from my lan (10.0.1.0/24). I would recommend removing the extra interface and also changing your lan to a /24 network.
If you are still unable to connect to the modem, maybe a static route would be the best way to achieve the goal.
that's what i do right now but its too much of a hassle every time enabling the modem wifi just to access it when you can just configure it to be accessible from the router
not really, but i read somewhere its better if the router and modem are on different subnets
modem subnet is 255.255.255.0 and router is 255.255.0.0 and i then configured modem interface with an ip on its subnet
oh, my bad. i always thought you can only have ips in the range of 192.168.1.x/24 on the 255.255.255.0 subnet and to "change subnet" is to change ipv4 subnet from 255.255.255.0 to 255.255.0.0, anyway i changed the router lan to 192.168.2.1 and now i can access the modem