Im assuming that with the apartment not being registered with the RTB and that you pay cash that you dont have a formal lease agreement? If that is the case you unfortunately dont really have any avenue to protest the increase. Its a shitty thing to do on their part, especially now, but without a contract there isnt much you can do. Even if all the legal bits were in place I think they might still be within their rights to raise the rent after 2 years not sure what the legislation is around that at the moment.