Screen Sharing Issue: Life After TeamViewer?

Hi everyone... this question will really only make sense of be solvable by users living in the Montreal area and who have Videotron as their ISP.

I previously used Macs convenient screen sharing function to help my elderly mum fix small problems on her computer from a distance. Her ISP is Videotron, and they do have excellent customer service, but are unable to help me with this. Videotron uses a router/modem called Helix, and it apparently creates dynamic IP addresses as well as adding an extra layer of firewall security. This now prevents the use of Mac OS's screen sharing ability. We resorted to TeamViewer, and they, quite greedily, have severely limited free use to only a few minutes. They are now blacklisted in my book.

Does anyone have an insight as to how I might reestablish a link using the MacOS screen sharing function? I have poked about the Helix app on my iPhone, and other than switch off "advanced security" measures, there are no other options to configure it to allow for the Mac Os screen sharing to function.

I am not a tech pro by any means, but an advanced power user - so I can do some "under the hood" stuff, but it is very limited. I prefer not to have to resort to the terminal.

Help!