Not sure if this is an issue with all certificate authorities but I saw this particular issue with GoDaddy.
Guest wireless captive portal is secured using a certificate from GoDaddy. When mobile devices get redirected to the captive portal they get a certificate error like the one shown below.
This does not occur on laptops though.
The resolution for me was to include the intermediate certificate in the issued certificate.
Open the certificate in a text viewer such as notepad. Also open the intermediate cert in notepad. Now copy the intermediate cert text and paste it after the issued cert text. It’ll look something like this….