Is your new Wi-Fi extender using both the 2.4 GHz and 5 GHz bands? Wyze Plugs can’t connect to 5 GHz Wi-Fi, so I often find it helpful to disable the 5 GHz radio/band/SSID on my router when setting up finicky “smart” devices. Once setup is complete, I turn the 5 GHz network back on. If you have the ability to control which radio(s) your extender uses, I’d consider trying that to see if you can get your Plugs connected.
Edit: If it’s not possible to control that on the extender itself, then just do it on the router (or whatever you’re using for your main wireless access point (WAP)). The Plugs are portable, so just set them up close to the WAP when the WAP is a 2.4-GHz-only network, and then move them where you want them and reset your network after you get the Plugs connected (assuming they connect).