If you want to debug this further, you could open the same page in a different browser (try installing one from scratch), on the same computer over a different network or on a different device (your phone) on the same network. Then check whether the certificate you get is the same, or a different one. This might tell you whether your ISP is to blame (it likely is not, this would be unheard of for that ISP).
Alternatively, do you have any other software on your computer that might tamper with network connections? Like ad blockers, VPN stuff, browser plugins, software from your employer to access some internal network, ...?