Free sessions to older versions of TeamViewer

I support my old uncle (99, half blind, half deaf) for his basic computing needs (installing stuff, &c). We were both on TV 14, things were working fine. Unfortunately, we had to install TV 12 so he could get support from a professional who is still on a (paid) version 12 license. This works, but now I can no longer connect to my uncle. I tried installing TV 12 (on two Macs, in one case a first install), but I can't connect : "The remote TeamVIewer is running an old version which is out of date".  I can understand why the company wants all free users on the latest version. But here, it's a show stopper.

What are my options ? I could get a license, but this is quite expensive since it seems I'd have to buy a minimum of a year. Any other option?

Thanks!