iPhone 11 14.2 disconnects from wi-fi when put to sleep

My wife and I have identical iPhone 11s, we both have wi-fi turned on so phone will backup at night if plugged in. Mine works just fine but hers disconnects from wi-fi every time her phone goes or is put to sleep. This means her phone won't backup. My Router is a Netgear Orbi24 and our provider is Xfinity. I can't see that either of these could be the problem, since my phone works fine.

Any comments would be appreciated.

Thanks,
chubbles