Sorry, I couldn’t reproduce this issue.
I tried change/add/remove certificate in NethServer and in FreePBX, moved certs away from /etc/asterisk/keys/integration and changed system times but no luck.
After removing the certificate in freepbx HTTPS still works.
Is the message still in dashboard? If everything works, I’d just ignore and click it away.
You may also try removing the certificate in FreePBX certificate manager, create a new one, make it default and check if something changes in dashboard between the steps.
So I think if you do not use WebRTC or SRTP you can safely ignore the warnings. There’s also a mini http server with freepbx (in advanced settings) but it’s disabled by default, maybe the certs are used there.