Many thanks for this brilliant post. It enabled me to get part of my network (192.168.19.0/24) working until I eventually figured out why my VPN router couldn't connect in modem mode (it had a node name set on the WAN port and the Hub 3 doesn't ignore it as the Hub 2 did).
I ended up getting a proper wireless router in the end too. For a while you could trick the 'Super' [sic] Hub firmware into working properly and allowing the network base IP address to be configured, but that tweak seemed to get broken in later firmware updates. Annoying though it is, just accept that it's a pile of crap and get a proper router and use the Virgin kit as a modem.
Just an fyi. My Superhub 3 was delivered today. It is hardware version 10 and software version 9.1.1811.401. I disabled WiFi and DHCP because my mesh WiFi and home server handle those anyway. Then I applied just the IP address and end session commands from the post on page 2 of this (mega long-running) thread and it worked perfectly for me, even though the IP address I applied is in the 10.50.x.x range, which did seem like a longshot to be honest.
So the workaround is still valid. Many thanks to @Eddy29
I tried setting the hub3 router (hardware version 11, software 9.1.1912.302) to 192.168.1.254 via those commands and the dhcp range to 192.168.1.10 - 192.168.1.240 (to match my old ISP router)
The IP address change worked fine, but the DHCP address change did not apply - which is obviously no good!. I was able to get onto the router's web UI page on the new IP after manually setting my IP settings then. Interestingly the web UI would appear to accept the previously unconfigurable 192.168.1 range then - but it would never actually apply.
I decided to just factory reset it for now so I can have the DHCP server work - even if its not the range I want it.
Maybe if we could work out how to change the IP range thats been reserved for the guest network we'd be able to use it for the main network instead? Oh well. I'll probably just have to reconfigure the few devices I've got on fixed IPs. The wifi will get disabled on the hub3 anyway as i have some BT whole-home discs to use instead (but those dont work as a router - so I still need the hub3 to do that, or I guess buy another one and use the hub3 in modem mode...)