Someone trying to isolate you from other people is a red flag (in both personal and professional life). Quite likely, they are telling you lies about other people, telling other people lies about you, or both. More reason for you to keep some informal communication channels. You want to know what your boss might be telling about you to the people you are not supposed to talk to (and therefore you are not supposed to find out). And even if it turns out that the answer is "nothing", having more contacts is only useful.
Realistically, how much can your boss actually prevent you from talking to others? In a different year, I would ask about coffee breaks and lunches, now you probably work from home... either way, there should be a chance to talk to someone. You probably already talk to some people as a part of your work. Add a bit of extra time for socializing. I am not good at small talk, but mere "by the way, how are you these days?" can often start an interesting conversation. The topic does not matter much, the important part is the mutual feeling of "it is okay to talk to each other".
After you are comfortable talking to each other, you might ask how is the work in their department, and whether there might be an opportunity for someone with your skills and experience. (See Pete W's comment.) Don't complain about your current situation, just express a desire for "professional growth", or something that sounds positive and professional. There is probably no good future for you under your current boss; the best you can do is walk away.
Also, update your CV. That doesn't mean that you should leave. However, being ready to leave if necessary, and knowing that you have other options, will increase your confidence and may improve your negotiation.