Sorry fljoemon, yes I was writing about android (and I modified my post)
If you're on Apple hopefully you've been syncing with iTunes on your computer; It's been a while since i've used iPhone but the old version of iGrill (3.2.2 for example) should be on your computer. Find your iTunes folder, then go into MobileAppsBackups, then MobileApplications, and then there should be a bunch of ipa files, and try to find the one that looks something like igrill 3.2.2.ipa or the like. Then delete the app off your iphone, and double click on the 3.2.2 file and sync with iTunes. I'd also suggest if you can turn off auto-update or iTunes will put the new version back on your phone the next time you update.
I have to apologize but it's been years since I've used an iphone, and furthermore I use a windows laptop, so if you're on an iMac or if they've changed iTunes in the past few years then the above may not be entirely accurate. I hope it helps.
I know how frustrating it is that Weber have chosen to totally and completely screw up their app. Idiots. I even called them the other day and complained, and the goober who took the call (admittedly not a tech guy I'm sure) said that the engineers at weber don't have any of the old versions of the apps. Utter nonsense. Fools.