I thought from the wording of your first post that you had already paired with your Netgear and that you were getting 31Mbps as against 21Mbps with the hub and all you want to know if it was OK to do that.
I don't have the discs so I can not say if they can be paired with a non BT router but I am sure somebody else will be able to advise you but if not why don't you just try it.
Can I pair it with my netgear though? It’s the black disc
I think you will find that the smart hub 2 forms the first (master) node in the mesh, and will not work with another router, but there is no harm in trying.
If it does not work, then you can always return the disc, as its only rented.
The white whole home discs will work with any router.
I assume your SH2 doesn't have a WAN port so you can't connect it to your Netgear running in bridge mode. So the alternative is to use the SH2 as an additional wireless access point - turn off its DHCP and make sure your Netgear is on a different IP address (e.g. 192.168.1.1 for the Netgear and 192.168.1.254 for the SH2). Connect the two routers via LAN ports and disable wireless on the Netgear. You may now be able to use wireless on the SH2 and black disk.