iPhone 6s - strange connectivity issues

My mother asked me to take a look at her phone since her WhatsApp app stopped working today.

She has an iPhone 6s. The first thing I noticed is the WiFi icon does not show up in the status bar at the top anymore. However, in the WiFi widget when you pull up from the bottom or when you look at the WiFi settings, it does appear to be connected to WiFi. It's just not showing the icon.

Whatsapp just shows a "connecting" status at the top of the screen when you enter a conversation. When someone attempts to call by voice or video it doesn't ring. And if you try to call out it won't ring on the other side.

I noticed some similar strange behaviour in at least one other app, a game, where when attempting to connect to facebook it just seems to get stuck but neither succeeds nor lets you log out.

I tried enabling and disabling WiFi, resetting the networking settings, uninstalling and reinstalling Whatsapp, and finally resetting the phone to factory settings and restoring from the iCloud backup. These steps didn't change anything. There's still no WiFi icon. Whatsapp still won't communicate and there seem to be some other connectivity issues as well. But it's not a total lack of WiFi connectivity as some apps like Safari seem to work fine.

I don't know about the WiFi icon, but at least Whatsapp was working fine as of a day ago or so.

Does anyone know what's going on here?

Have there been any recent OS updates that may not be behaving with this model phone?

Thanks!