Hi @dennisc Welcome to TPG community, a mesh system is best used to handle all the WiFi in your network, this means you should be disabling the built in WiFi in the VR1600v.
There was maybe a misunderstanding with what you were trying to achieve with the VR1600v as it doesn't have mesh functionality built in. It can either be used on it's own or with a separate mesh system, you won't be able to combine the VR1600v's internal WiFi with a third party mesh system and have just one SSID.
The best way is to use a separate mesh system to provide all the WiFi and hence a single SSID, this system simply connects to a LAN port on the VR1600v.
I'm using a TP-Link Deco M5 system with my VR1600v router and it has been excellent, with on average 6-7 client devices connected most of the time. Great coverage and reliable performance, there are many brands and models available on the market, I'll have a look at the Tenda Nova MW6 system to see how it compares the the Deco M5 on paper.