It’s not too difficult to establish a Signal account from a burner number from a prepaid sim card. I currently have a Signal account tied to a sim not in my name. Getting a burner with cash is an option. Or, if you’re lucky enough to live near a payphone and can gain access to the number, you can activate a signal with a phone call.
Just to clarify, Puerto Ricans are US Citizens and can vote for President if they are residents of any of the 50 States. It’s only Puerto Ricans living in Puerto Rico (or other citizens living there) who cannot vote for President because the Constitution only grants this right to US States not federal territories.
2021 Census data captured 5.8 million Puerto Ricans living in a Presidential voting area but only time will tell if this made an impact.