ja, the problem is that if you had a normal router or if the shub was a proper router then you would be able to do it no problem because the features you would need to enable and disable would be there for it. The problem is that the shub isn't a proper router and can really only do one thing.
By trying to connect it to the coax/VM network you are essentially asking it to serve as a second gateway and have it's own ip routable ip address and VM won't let you do that. If you somehow managed to get it connected and working with the other router you would still have a messy network and double natting.
The best thing you can do if you feel you need the extra wifi coverage is to put the shub in modem mode relieving it of all routing responsibility and buy a proper which in itself will probably fix your problem because it will be better and if it doesn't you can always get a second and the two proper routers will have built in features that will work together. As an example, my primary router is upstairs on one side of the house and I have a second downstairs on the other side of the house which is in access point mode. There are options on routers which you can tweak (like disabling dhcp) which can make routers mimic certain roles but I don't have to do anything on the one downstairs. Because it is a good router it has certain preconfigured functions. When I put it in access point mode, all the settings are changed and it knows what it is doing and is hassle free.
This would be the bees knees for you and is what I have got and uses AI Mesh which is something you may also want to look at.
https://www.amazon.co.uk/ASUS-AC1900...us+ac68u&psc=1