Error in our local Survey Solutions server

I, on behalf of our Institution, have setup a local Survey Solution server. I did two for different surveys.
In one, htts://xxxxx.com works well but http://yyyyy.com has some issues.
We (me and team) have setup the survey users and imported the questionnaire from Designer.
On mobile devices, we are getting the following error from this second server:
"NO CONNECTION TO THE SURVEY SOLUTIONS SUPERVISOR. PLEASE MAKE SURE THAT THE WEBSITE IS AVAILABLE”. Our server is on win 2012.

On both servers, we have generated a self SSL.

What is the issue? Please assist.

The general recommendations for this kind of problems are outlined here:
https://support.mysurvey.solutions/interviewer/troubleshooting/synchronization-problems/