As a technical question: no, if everyone you want to peer with wants to kick you off, you can't be on the "internet" in the regular sense.
Signal was funded by Radio Free Asia, which historically got past the general form of this problem by broadcasting radio stations from right outside the borders of a hostile country. St. Maximilian Kolbe did the same with amateur radio.
I think the more interesting question is some blend of technical and political: how do you get across an idea that people aren't exposed to? Radio Free Asia/Europe/etc. and Kolbe all faced this problem, as did the samizdat publishers, the organizers of the Underground Railroad, etc. You have to be creative, but it turns out that getting ideas across is not an unsolved technical problem, it just requires a bit of work. If the goal is to make people aware that an alternative to your authoritarian government exists or that an organized scheme to take you to freedom exists, you can communicate it, even without anyone else's infrastructure.
Neither the purpose nor the effect of Trump's ban is to prevent people from knowing that his ideas exist. They are well-known and popular. It denies him the use of mass media (and even so, newspapers and TV stations haven't banned him), and it disrupts the ability of people to coordinate. Regardless of the merits of the ban, I think it's pretty clear this is a different sort of problem, and yes, if you want to use wide-reaching, high-bandwidth, reliable infrastructure that other people built, you need to make them okay with you using the infrastructure.
An analogy can be made to spam. Obviously everyone knows that you can lose money by wiring it to some random dude with a foreign bank account; the "censorship" of 419 scams does not in any way restrict what people know about, and spammers get denied the use of infrastructure if they make the people who built the infrastructure unhappy.
In terms of technical means, I'm not sure how to permit an existing, major political party to have access to infrastructure however they want without also, by implication, permitting spammers to have access to that same infrastructure however they want. The only distinction between those is human judgment.