@Tryptophane: the first thing to check is ‘Last Successful Weather Call’ time stamp. You can find it via ‘Weather Diagnostics’, or on the controller’s LCD screen directly by pressing B2+B3 (the same way as how you press Ctrl+A). If the Last Successful Weather Call time stamp is synchronized with the Last Weather call time stamp (i.e. within a minute from the call), that means the controller is receiving data from the weather server correctly. If not, it could be a DNS issue as you asked.
Particularly, if you use static IP, the firmware currently assumes the DNS server is the same as the router / gateway. This can sometimes cause issues. I recommend you to turn it back to DHCP, and use the router’s DHCP reservation feature to achieve static IP instead.