iPhone 11 Pro Max has trouble connecting to, and staying connected to, known WiFi networks that other devices have to issue with.
https://discussions.apple.com/thread/250659429?page=1
Here's how to fix it based on the comments on the Apple Support Thread. It worked for me, I hope it works for you
First, Your iPhone has VPN. (Virtual Private Network)
That was news to me. I know you used to have to buy a subscription to a VPN service. Now, VPN is in SETTINGS.
Turn off VPN.
Turn off your router.
Turn off your iPhone.
Wait a minute.
Turn on the router.
Turn on the iPhone.