Go to android settings > apps > special app permissions (the last option) > alarms and reminders
The app causing the issue should be there, among others
Some apps that do some kind of scheduling display the alarm clock icon.
Do you have any app set to do something at a given time? Maybe an automated backup or some kind of automation?
Even the version number looks like an IP address
For your usecase a pi should be sufficient. You can go with a pi 5 8GB and a docker install, so you can host more stuff later. I would recommend an m.2 head with an SSD instead of an SD card though. Much faster and more reliable.
You can also just block YouTube in your unifi firewall/router for that one client.