I can hear my mother via a remote camera feed but she can't hear me, neither via the camera nor her phone. She has dementia and can't operate her phone to read normal SMS messages. Basically I need to be able to have a conversation with her, such that I "text" and she "reads", via this as yet unknown device, while she "talks" and I "listen", via the camera. She has an NBN broadband internet connection.
Sometimes we need to go back to basics that had worked for many decades in the past, the landline telephone. Ok, we don't get to see that person on a screen, but that didn't stop our great-grandparents and great-grandaunts/uncles from communicating with each other on a regular basis. Heck, I remember back in the 1970's and 1980's hand writing weekly letters to my parents, who lived out-of-state.
Plus landline telephones can save us seniors $$$ in the long run. My landline phones are a decade old and still does the job perfectly, no constant updating or re-charging :)
the reason I ask is if she has dementia can she read and understand the written word?
If she does not have dementia and can read and understand the written word then you can display text messages on a Smart TV screen. I would imagine that the printing could be made as large as needed.
I wish you success in finding something that works, just saying you should have tempered expectations and don't invest too much time and money on a fix that, even if it works, it may only work briefly.
You can contact organizations for hearing impaired/deaf people and they may be able to give you some ideas on how they do it.