If you've encountered this issue, here is how to solve it! 

Warning screen:

Profile cannot be linked from main menu on PC due to possible loss of data from other Bohemia Interactive games.
Profile cannot be linked from main menu on mobile guest account because discarded account would not be accessible by any means from that point on.
To login to your already linked existing account on mobile from guest account you have to link your guest account first.

Step-by-step solution: 

1. Open your profile by selecting the avatar icon and then profile

2. Select link - create new account - fill in all credentials (new!) and confirm

(fill in a brand new account information, you will be able to go to your original account later) 

3. Now you can change your account on your profile screen. 

4. Now you can login normally, just like you would when you start the game for the first time.