After resetting iphone 6+ it's requiring me to update my phone in order to install apps, even though I downloaded one 3 days ago with no problems

is there a way to fix this? usually It gives me an option to download a previous version suited for my device, but after resetting + reactivation, it doesn't. Is it just terrible coincidental timing on my part and they removed the previous versions today? Updating isn't an option for me. Specifically one of the apps in question is Instagram, downloaded it no problem a few days ago, yet now it refuses. This pretty much takes the smart outta the phone which kinda dumps lol, I can practically only make calls. so I'm trying my best to find a solution :(