Sudden TLS/SSL RestSharp failure on Win7 comps

I have C# WPF net framework 4.7.2 app running which has been working for about 18 months on 1200 computers.

Just this morning I have had support calls as a number of computers cannot load our app.

In debug we are finding that RestSharp is having issues getting a secure channel:

The request was aborted: Could not create SSL/TLS secure channel. no expection data

After more research I've found that the computers seemingly affected are all WINDOWS 7 computers.

I have this in my restsharp code:

 ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

(just in case someone was going to mention it)

The issues just started a few days ago but I cant seem to find the solution.

When the user loads our website it works fine, so it seems to be related to either RestSharp or perhaps a net framework update??

Can anyone please advise of how I can fix this or diagnose further as I'm a startup and really can't have these systems offline right now.

I do appreicate Windows 7 is OLD but this has been working for 18 months so my clients won't accept me saying "get a new comp" as the answer.

Thanks very much for any advice.

I FIXED IT!!

In WHM SSL/TLS configuration: Fault SSL/TLS type, RSA was still selected because CPANEL apparenrly auto ships with that as default.

I have no idea why everyhting suddenly changed, but i achanged to RCDSA P256 and refreshed the SSL and now I'm still A+ on the SSL checker site but windows 7 works fine again!!