I’ve spoken to someone who is deafblind before. They had an interpreter that would fingerspell onto their hand by touching it in certain places and making gestures. They would then fingerspell their answer back to the interpreter, and the interpreter would speak what they replied.

A lot of people don’t know that you can use computers and phones with a refreshable braille display. Some people who are deafblind can also read braille, so you could send them some text to their phone/computer, and then they should be able to read that text with a refreshable braille display. They then could reply to you by typing on their normal or braille keyboard, meaning you’ll get a reply in text.

https://en.m.wikipedia.org/wiki/Refreshable_braille_display