Try flushing the DNS cache on your PC:
Clear, Reset, Flush Windows DNS Cache
Also try a disk clean and delete temporary internet files.
You could also try switching to a different DNS lookup:
How to Switch to OpenDNS or Google DNS to Speed Up Web Browsing
That is weird. I'm afraid I've no answer to that... if you can;t get it on the 4G network as well as over normal broadband then it sounds more like the site is blocked for some reason.
Is your 4G provider the same as your normal internet? Have you tried accessing it from a different network/friends pc etc.