Rated 5 out of 5 stars

Loved it, used it. But since yesterday I get a message that the certificate has expired october 2nd

This review is for a previous version of the add-on (1.0.9.8). 

I contacted Telenet and will correct the module if they do not renew the certificate.
In the mean time, you can go to https://telemeter4tools.services.telenet.be/TelemeterService and add a permanent exception for the certificate (if you're using Firefox 3).