Connecting to second modem WiFi

My PC is plugged into a second modem which then plugs into the modem where I receive my internet. If my phone is connected to the wifi of the second modem all works well, but if I am connected to the wifi of the principle modem I cannot locate the PC on my phone app.
Is there a solution for this?
Also, if I get an extender for my modem and connect to that WiFi, will I have the same problem?

Well both modems have their own local networks, which means both devices are in different local networks, while A can access B, B can’t access A, what you could do is search a way to link both devices to the same network.

Good Luck!