Well done! That's working as intended. When there's 24 hours we'll be able to see what's going on (if anything).
If you can wait until this time tomorrow before playing with the new router that would be good, because the BQM relies on your IP address (the set of numbers that act as the "digital postal address" for any device connected to the internet), but when you switch between your hub's normal router mode and modem mode the IP address changes, and such a change would interrupt the BQM working unless you edit for a changed IP address.
If you can't wait to try the new toy, then simply delete the existing BQM, and as soon as the new router's up and working with the hub in modem mode then repeat what you just did, and a new BQM will pick up the new IP address and start tracking. If I'd bought a new router I wouldn't wait!
Surely if it was congestion on the network, we'd either all have no internet connection or we all would but it would be slow. This is more like the Hub decides to only let certain devices connect when it feels like it.
Actually VM network congestion is usually associated with erratic response times and poor latency (ie gaming, video calls and Teams/Zoom freezes), but I was just wondering about this because you mentioned staff had suggested this might be the problem. That and certain other things would be apparent from a 24 hour BQM trace, but there are other possible causes that wouldn't show up. Possibly it's just a wireless coverage issue, and if that's the case a mesh system should improve things.