As for me personally, I had believed that RPM was in an alternate universe, but the RPM rangers being present in the mega teamup next year is giving me doubts.
That seems like it would be easy to explain. The Red RPM Ranger already traveled dimensions in PR Samurai (I only saw it once, but I'm 99% certain that they did say he was from another dimension, and not just the future). I'm assuming some ranger or ranger ally in the normal universe has the tech to send a message to the RPM dimension. So when the rangers in the normal universe get to the War, someone calls the RPM dimension for help. The RPM rangers grab their morphers and get to the normal universe either the same way RPM Red did (I don't really remember how they explained his way of dimension hopping), or some other way thought up by Dr. K or a smart person in the normal PR universe.