Simple. If you haven't already contacted Bandai support and notify them that you're in need of a transfer code due to facebook discontinuation, fill out the form as much as you can. Get your player ID, current level, memorizing 5 characters you have, complete name, rarity level. When you started playing, last day logged in. Everything on the form as much as possible. After a few days, they'll possibly ask for more info or at least notify that they're looking into it. If successful, they'll send a transfer code. Once you have this, LOG INTO THE OFFICIAL APP AND LINK WITH GOOGLE. Then you can return to the modded app.
I made this mistake. I didn't log into the official app and linked to google first. I went right into the modded app, now as long as nothing happens to my phone, I'll have my account.