This could be related to the UPNP sensor in HA? Try disabling it?
Iām not using this sensor. So Iām affraid this isnāt the reason its failingā¦
I donāt think itās related to the issue, but Iām running HassIO on my Synology NAS using a package from this thread: Hass.io on Synology DSM (native package)
Doesnāt look like youāre the only one who is having issue, saw another 3 similar posts popped up. The recommendation was PiHole and dnsmasq.
Just curious, what if you use Google nameservers, instead of your ISP DNS? You could plug in Google nameservers on your router. The reason Iāms saying this because I host my own mail server. When Iām at home, it understands FQDN. Once I switched to ISPās nameservers, it doesnāt. It might work in your case.
Also, I donāt speak your language, but if I understand correctly that ssl error/warning is normal. Itās telling you that the issued certificate could not be verified. Since you know it is your server and not being directed to another server, itās harmless.
I can try using the Google DNS on my device this evening. I canāt change this in my router because almost everything is blocked by my ISP.
I know the SSL error is normal, Iām just trying to give as much information as possible 
I am using hassio on a raspberry pi and I also use a Telenet wireless router. I donāt have any problems.
Did you try to disable upnp on your Telenet router ? You can find the setting in the advanced settings of your router.