Yes you absolutely can use a 3rd party router connected to the SH2. One way is to just use access point mode like above, but if you want to take advantage of all the features of your new 3rd party router:
1.) Connect your secondary router's WAN port via ethernet to one of the LAN ports of the SH2.
2.) Then make sure to turn off the DHCP, NAT, WiFi of the SH2. Give your secondary router a static IP, then put that IP in the DMZ of your SH2(To avoid any double natting issues), and you should be good to go.
3.) If you want IPV6 just make sure to enable "pass through(bridged)" mode on the IPV6 page of your secondary router.
(If you want to use your BT TV box over WiFi, make sure to get a router with IGMP snooping)
4.) Reboot them both for good measure and then everything should be working.
It's a little complicated to get set up, it took me 3 tries, and I even had to wipe the router and start over again because I messed up, but it's well worth it in the end. Now I have WiFi 6 and a DHCP server that won't start to choke after a while, and I get to keep Digital Voice working too, because the SH2 is still taking care of that! 😊
