@JohnConstableRA I have an #Android phone, and I never used the generic Mastodon app but started directly with one of the alternatives (currently #Tusky, thinking about a switch to #fedilab)
From what I understand is that many #iPhone users have also switched to one of the other apps (basically to anything but #Mastodon), like #IceCubes, #Toot, #Mammoth, #Mona or #Ivory. Some are free, others paid. You could check the posts from @feditips.
Good luck for solving your freezing problem.