Handle T-Mobile and Deutsche Telekom cases properly

[ameshkov]

T-Mobile relies on their own custom DNS server in order to provide connectivity with IPv4 servers over their IPv6-only network.

Domain names are resolved to some private IPv6 addresses which are then re-routed to the real IPv4 addresses (invisibly for the user).

The problem with that is that you simply cannot use a custom DNS server in that case. At least we need to warn T-Mobile users about that.

We need help from T-Mobile users who have this issue

[dnovitskyi]

We've got something that looks like the same but is related to Japanese carrier DOCOMO.
Logs and all related information available under 543636.
Here, however, the problems only occur when using LTE networks.