I have a 2620Ln on a Vodafone SoGEA VDSL connection and it keeps dropping the line.
The Internet circuit from Vodafone is fine when it is connected to a Draytek 2762 so I know the internet circuit is OK.

The 2620Ln drops the WAN connection randomly, there is no reason and no specifics that seem to make it drop. It is on the latest firmware 3.9.8.8_BT and DSL  Modem Code is Default.
The cabinet it is in is cool and not getting hot. 
It drops the WAN and doesn't revert to the 4G circuit until the RJ11 is unplugged, then it works fine on 4G mobile connecttion.

Other than a faulty Draytek2620Ln I cannot see what is wrong....any suggestions?

Hi,

Check your ‘WAN Connection Detection’ settings on your RJ11 connection; see page 61 & 62 of your router’s manual.

When this parameter is correctly set, the failover should kick-in and your failover alternative connection should take over.

It's set to the default "PPP Detect"
With the ongoing issue we have seen it drop to the 4G backup but not always.
Openreach are now saying no problem with the VDSL but still the Draytek 2620Ln drops the VDSL WAN with no rhythm or reason