Tapatalk forum app for iPhone is now free.
I participate in a lot of forums:
and lots of others
I first found tapatalk a while back and I use it every day.
a few of the features that you’ll like:
Tapatalk remembers the forums you use and will sync your active forums across devices
It doesn’t sync the login details, so you will have to login on each device, but you don’t have to hunt down each individual forum or type in all of the details on each device.