Remote doesn't mean devoid of normal human behavior. Suck it up and call them on the phone or do a video call and take care of it. That is the proper way. You will send an e-mail as a follow up to confirm all the details and off-boarding information. But you should never (under normal circumstances) have a persons first interaction of termination be a text or email. The one time it may be acceptable is someone who is ghosting you and playing games, then, all bets are off -- if they act like a child they can be treated like one.
For reference I have worked remote and employed my teams remote for over a decade, not once have I ever used text, email or slack etc to fire or reprimand someone. I always use phone calls, video chats as the primary means of human to human discussion. I may use slack or email as follow up means on a conversation but not as the first conversation of a serious topic like their employment or job performance etc.
Would you really want your employer sending you an e-mail firing you? Would you like your significant other to dump you via text after say a year long relationship? If you are in the minority and like that idea, recognize most people are not like you and you need to conform, not them (not saying that to be mean, just to help you recognize it maybe). Being an employer is a responsibility to treat people with respect, dignity and professionalism. If I heard a startup or company was letting go of employees via e-mail (outside of extreme circumstances) I'd dismiss them as unprofessional and likely never consider them a serious business.