iMac Safari slow with strong Wi-Fi signal and T-Mobile gateway

A neighbor recently switch to T-Mobile but failed to tell me that it was their wireless gateway and not a fiber land-line connection. (T-Mobile purchased our fiber provider, Metronet and I wrongly understood that they still had a wired internet connection). The problem we had was with an iMac in the basement that had a strong wireless signal from the router upstairs but not enough bandwidth to run Safari reliably. If we turned Wifi off then on, Safari would connect to the website but then slow down again. None of the iMac diagnostics pointed to a bandwidth issue. We discovered the problem when we connected the iMac to an iPhone cellular Hotspot and Safari ran fine.