Phone connects to WiFi for a brief second, then disconnects instantly.

My grandma just came up from DC to “babysit” my younger siblings while my mom went on vacation. On her first 2 days here at our apartment, her internet worked fine. Now, starting only today, her phone is having problems I just can’t solve on my own. Basically, her phone disconnects from our WiFi network instantly after connecting. You simply press “connect” after typing in the WiFi password and you won’t be connected for even a minute and it’ll just disconnect. Almost as if the phone is rejecting the connection. I tried sharing my cellular with her to see if it was a problem with our WiFi and not her phone, but it rejects that as well. I’ve tried almost everything possible: resetting network settings, restarting her phone, restarting our router, forgetting the network, nothing seems to work. I don’t know what’s going on or how to fix it. Any help? She has an iPhone 8 Plus running on iOS 13.5.1.