I have a mobile account that's linked to my apple ID. When I'm logged in, it gives me the option to tap the 'link account' tab but says I already have an account/email linked. When I use those same credentials on Desktop, it says I haven't linked the account.
"When I'm logged in, it gives me the option to tap the 'link account' tab but says I already have an account/email linked"
Only one RuneScape account can be linked to your Apple ID.
Can you see the
'link with Apple'
button or the
'unlink Apple'
button?
If you can see the
'link with Apple'
button, you are logged into a different account, not your Apple ID linked account.
"When I use those same credentials on Desktop, it says I haven't linked the account."
If you created your account via Apple ID, the email you have set with your Apple ID is
not
automatically the login for your RuneScape account and your RuneScape account might not yet actually have an email login.
To add a login email to an account created through a third party login, use these instructions:
Add a login email to third party account
It is possible you have accidentally created two accounts, one with Apple ID, one without.