When I updated my iPhone 8 to 12.2 (16E227), it broke the WiFi and now apps like YouTube can't connect and often times I can't even load apple.com in Safari. Podcast app also fails when streaming. When I restart the phone then it works for about a minute. When I turn airplane mode on and off, it works for about a minute. All apps are up-to-date.
However my iPad with iOS 9.3.5 works flawlessly. My Samsung laptop works flawlessly. Also when I turn WiFi off on my iPhone and use cell data, it also works.
A user said it wasn't possible because Apple "stopped signing" old versions, but that doesn't make any sense. A signature is a static sequence of bytes. If you are familiar with cryptography, a signed file is just another file that uses the signer's private key. You don't "stop signing" something because signing a file is a one-time event, not an ongoing process. As long as you have the file, the file's signature, and the public key, you have a signed, verifiable file.