Had a similar issue with a Moodle 3.4.nowhighest on a server that did have partially configured comodo global certificate for the domain. Not exactly your situation as you've described, but still had to do with the 'warning' seen in the config/setup of mobile access ...
Did you set up a self-signed certificate or did you acquire one from a Certificate of Authority?
Any of the following:
should work but each may have it's differences in how to install, etc. For example, many have separate instructions for the entire process for Apache on Linux systems. Pick the directions for your choice of 'poison'. ;)
'spirit of sharing', Ken