Upgraded to wifi modem and can't reconnect extender

I guess the issue is moot, now. I traded in my Netgear unit for a Linksys one, which is compatible with my new modem. The only way I was ever going to be able to use the Netgear one appeared to be to plug in my old router again and set up using that, which seemed pointless given the new unit does both. The WPS button on the new modem was disabled, apparently due to security issues, according to my ISP. This, plus the fact that the 2G/5G networks wouldn't show up for me, rendered the Netgear unit useless.

Thanks to Babylon5 for trying to help, anyway.