How to connect to a modem locally?

Question

proxy

Good evening everyone.
I have several 3G modems, when I tried to make mobile proxies out of them, but unfortunately the attempt was not successful, I'm not very good at this and "knowledgeable" people told me that it would not be possible to raise mobile proxies on such modems.
But I came to the crutch method, to make virtual machines and connect modems to them, like the main network. This is my question, I need to run the software I need on each virtual machine and specify a proxy in it, this is a mandatory item, how can I specify my main connection instead of a proxy (that is, enter a local IP there or something like that)? I would be very grateful for the hints, because I do not understand what information to read and where to dig.