Unless things have changed what the app shows is the "sync" rate, which is a raw connection between your modem/router and the OpenReach cabinet. In almost all circumstances the sync rate is going to be higher than the actual usable data rate due to overheads in the data protocols used. But ideally, that overhead shouldn't be more than a few percent!
In order to use the speed test sites properly, you need to follow their guidelines such as ensuring the device doing the testing is the only device on the network, and that it is connected via an ethernet cable. If you have other devices using your bandwidth in the background, or ESPECIALLY with the VF router you are trying to perform a speed test over the 2.4GHz band on the VOX2.5/HHG2500 you'll more than likely end up bottlenecked by the link between your device and the modem.
If none of that applies, and you are doing everything right, then I'm afraid you may need to call VF tech support - in which case I'd make sure you have a flask of tea and something to nibble on while you wait.... and wait.... and wait....
One of the advantages of having your own router is that many will be able to tell you what your actual data rate is, as opposed to just the sync rate!
