Remote Support for Doro phone

Hi all, I am trying to supply remote support for my father-in-law who has a Doro 8050. It has Teamviewer Remote Assistance built-in and Doro themselves are able to connect and control the phone. I have a licenced copy of ov TV ver 10 but if I try to connect I am prompted to add a Remote Assistance licence which I am reluctant to do for a 'one off' situation. I can see that the remote user  has indeed logged on. My solution therefore was to download and install v15 on a seperate machine (I have tried this on both Mac OS 15 and Win 10) but when I try to connect I get a message saying that the remote version of TV older than the one I am using and that the remote user should update. As I understand it TV is backwards compatible for such support and I am reluctant to do the upgrade as this may mean that Doro themselves will not be able to connect as I understand they use v10. As I said I have tried different OSs, different wireless access points and even used my own phone as a hotspot to eliminate my office network altogether. Help!