I haven't worked out all the kinks, because Tapatalk seems a bit lame in this respect. But being a firm believer in 'a picture is worth a thousand words' - looks like you have to go into Settings, uncheck 'Auto Favorites Forum' and from then on when you choose 'More' it'll be one of the options shown. Now if it would just work... [Edit: if you long-press on the forum that you want and choose 'Subscribe' that will do the trick. Don't forget to refresh your Favorites...]
Sent from my Sinclair ZX-80 using Tapatalk