Only citizens residing in a state for the majority of the year can vote for federal elections. Basically you need a senator to vote federally. Hawaii and all other states were the same way when they too were territories. All PR needs to do is vote for statehood and then I guess the political shitshow starts as well as flag redesign.
That exception was the 23rd amendment: https://en.wikipedia.org/wiki/Twenty-third_Amendment_to_the_United_States_Constitution
Puerto Ricans as an example don’t meet the same conditions e.g. paying federal income tax. Hence statehood as their option to representation.