Changing your DNS doesn't stop your ISP seeing what sites you visit unless you use a VPN. DNS just resolves an address to an IP. Your requests (with destination IP attached) will still pass through their network gear and be able to be logged should they wish to.
You could buy a cheap router, put your modem in bridge mode, and set the Wi-Fi SSID and password combo to be the same as your modem’s and none would be the wiser. Then you’d have control over your network. That said, if you have the same modem that I do (from shaw) a cheap router will not have the same performance as the one shaw provides. But a used UniFi WAP would outshine the shaw provided modem any day.
I also did this, I feel it's better this way. Also the modem restricted me in so many ways. Now my cheap router gives me far more freedom and control. Forwarding ports, no problem. DNS change, no problem. Other SSID, no problem. A separate isolated SSID for visits, absolutely. And if a I change ISP I only have to connect the new modem and no need to change any of my devices.
Maybe as a workaround you could change the dns in your device’s settings. Not the prettiest solution, but should work. The advantage would be it then is used no matter what wifi your device is connected.