Craft 3 - login problem on mobile
By default, Craft requires that a browser has a user agent string and IP address for a valid authenticated session, and once that session starts, the user agent string must remain the same for the remainder of the session.

It sounds like whatever browsers are being used on the mobile devices aren't providing this.

You can alter this behavior with these config settings:

https://docs.craftcms.com/v3/config/config-settings.html#requireuseragentandipforsession

https://docs.craftcms.com/v3/config/config-settings.html#requirematchinguseragentforsession