Same problem here (Iphone 11).
Bought it in the US in November, started having issues in December. I got super mad because I was traveling and I had no local SIM card, so had to rely on WIFIs, and whenever I would get back to the hotels at night, my phone completely freezed and I had to struggle for minutes to try shutting it down. The screen was super slow in responding my commands... The reality is, only some WIFIs are causing this problem. There might be a bug in some routers that is causing it? Fun fact is, the Apple store WIFI is one of them. I first showed it to an Apple store assistant, who thought was a screen problem, so I got a new screen. Then after giving proof that it was still happening (through videos taken from my friend's phone), I got a phone replacement. Now I can say this is still happening, so the problem is seriously with the Iphone 11. I can never trust 100% my phone, example: I won't buy the bus tickets on it, as it might stop working if I have to show the ticket to the driver, or save credit cards though Apple Pay. It's ridiculous that this happens on a +1000$ worth phone. Please Apple, do something!