I started having the same issues with my Iphone around the time this thread was started. Running a free ad blocker has fixed it for me.
The problem was (other than the ad at that time being giant close-up of Trump stuffing some kind of food in his face) that the ad took up the whole page, and even when I could see a little "x" up in the corner, even with my non-sausage fingers, 9 times out of 10 rather than closing the pop-up, I missed the "x" and instead clicked the ad and was taken to another site.
So yeah, forum was unusable on my phone for a few days, until I downloaded an ad blocker.
Turning off your Iphone every now and then is a good idea regardless, but I don't think this ad pop-up has anything to do with that.