Hub 5 and DECO M5

Hi there, I've recently received a Hub 5 and 1Gbps broadband, all good.

My issue is that I've been using three TP-Link DECO M5s for several years with great success. My previous VM Hub was not delivering a consistent signal throughout the house.

I've set up the Hub 5 and connected the master DECO to it via Ethernet cable and everything works well... except... the speed I measure on the DECO network is slower than what I measure on the new VM network. 

I didn't find the need to set the Hub 5 to modem mode at first but, I did so to see if the speed of the DECO network connection improved. In modem mode, I could not even access the internet using the Hub 5 whilst connected to it with an Ethernet cable to my MacBook Pro – so I couldn't try the experiment as the Hub 5 wouldn't 'feed' the DECO.

I reset the Hub 5 and I'm back where I started, a blisteringly fast VM network and my pretty fast DECO network. Has anyone experienced this and or have any idea what might be wrong? Thanks in advance.