After the system upgrading, you will lose linked account(s) that do not belong to you. This is normal as unifi Mobile is now using a new 10 digits account number.
You can always link your account back, as long as you know your new account number.
STEP 1 : Update your myunifi app and open the app. Tap on Link Account.
STEP 2 : Fill up the details with new 10 digits unifi Mobile account number. This is compulsory or else you cannot proceed. Put nickname and category accordingly. Tap Next.
STEP 3: Double check the account number that you have inserted earlier. Tick the check box and proceed with Submit.
STEP 4: That is all. now you have successfully link the account. Tap on view account and you will see this account with your other account altogether. Now you can pay, or view the outstanding payment for that account.
That is it guys. Easy as pie, as long as you know the account number.