As a temporary measure, put the hub back in router mode. If you can flick the Archer C9 into access point mode then all existing wifi and devices should continue to connect OK. See what then happens, particularly with a sample device hard wired to the hub.
If you're still seeing poor speeds, then we can say that the problem is with VM's gear, and you can try and phone in as a speed fault, or wait on the forum staff to look in on this thread. If you try and get help with your own wifi gear connected, VM will automatically blame your equipment. Personally I'd wait on the forum staff rather than endure the slow, miserable experience VM continue to offer customers phoning in faults.