I am so fed up of trying to fix something when I have no idea what I am doing but hope that by putting the hub into modem mode and using the Deco will fix/stabilise the issues.
I suspect that's unlikely - an unreliable connection is rarely a wifi or router problem, it's almost always the cable modem connection to VM's network. The Deco AX3000 is a solid piece of kit, it may help with reach and coverage of the wireless signal, but I suspect there's an unresolved noise problem on your VM cable connection. We can conclude that it isn't a hub fault because you're now on your third hub.
What worries me is your earlier post where you mentioned being told that there was congestion on VM's network. If true that;s really worrying because VM's record of (a) honesty, and (b) competence in fixing these types of fault is very poor. Of course, the agent could just have been making it up (the offshore mumblers are particularly bad for that), but we still need to identify the problem because VM are failing to own and resolve the problem.
I appreciate you're frustrated with the crap connection and crap support, but if you'll indulge me will you try this: Setup a Broadband Quality Monitor
Don't get put off by the complicated sounding technical guff, just one step at a time, follow the instructions. Once you've set that up, post a LINK here to a LIVE, SHARED graph here and we'll see what's happening. Usually needs to run for 24 hours before we can draw reasonable conclusions, but the live graph will continuously update so you can post the link immediately. The forum members can then take a look and offer advice on what that shows because VM aren't owning this problem.