seragold I am beginning to wonder if comcast itself is unfriendly to grapheneos. I haven't seen these problems on calyxos on this comcast network but I defininely have problems with both grapheneos devices I have tried here.
I have nothing good to say about Comcast, but in this case the cause of the issue is definitely you and your settings.
If the issue would be caused by GOS, it would probably behave the same way when on WiFi and LTE, but it doesn't, which indicates that you messed with the WiFi setting, and especially DNS.
Just because there's a setting in the system doesn't mean that you should touch it or change it, especially if you do not know what you are doing.
Personally, I would start checking if the issue is in the system or in the browser. You can test other browser to see if the pages work or not. If yes, the issue is in the browser (probably usage of your privately added secure DNS). If not, the settings it is.
The cause could also be connected to your VPN (if you use it), Netguard (if you use it), firewall etc.