If you are still having a certificate issue your old Cert might be stuck. SSH with WinSCP into your PBX. Go to: /etc/asterisk/keys and .old your _account folder to _account.old
Now go back and try to request the certificate, it should be successful. While you are there don’t forget to highlight over the cert under default and check it as the default cert.
Then head over to System Admin and HTTPS Setup. Click on Settings and Select your LetsEncrypt cert then hit install. Wait for it to finish. You should now be able to connect from mobile. Mobile requires that a cert be correctly installed while desktop does not.