you edited your message while I was writing, so my guess right now would be the disabled IPv6.
You possibly have IPv6 DNS enabled, and the download tools (uclient-fetch, wget, ...) prefer IPv6, but then you have disabled the actual IPv6 connectivity, and the fallback does not work.
I think that uclient-fetch has no proper fallback from IPv6 to IPv4, but the full GNU wget has.
You might install the full wget manually.
EDIT: you might read: Unable to get opkg list update after sysupgrade (ipv6 connectivity)