Thanks both @SolanaFoo2 and @Kevin_McIntyre . That worked! Sofoo, the way you explained it really helped. I'd already read a bunch of post/articles, but it hadn't been as clear to me as the way you laid it out.
I have another easy win for you guys. Now that I've added in the link as text in the pop-up, I notice only the link is appearing in Field Map mobile. All the other fields in the pop-up have disappeared... And then, when I remove the text (which contains the link) in Field Map web/desktop, all the fields appear again in the mobile version. Field Maps on web/desktop looks perfectly normal, but it's bugging out on mobile. Both are online. Haven't moved over to offline yet. Yes, I'm saving in Map Viewer. I'll try starting from scratch and creating a new map/adding the layers, but if you know of another solution, I'd be keen to hear.