Do certain ports get blocked?

[#1]

Nice one!

[#2]

FWIW I have a TP-Link TD-8840T, I'll give this test a go as well when I get home. Thanks again for the suggestions.

[#3]

From a quick search,  it looks like the TD-8840T doesn't do NAT Loopback,  or (going from one of the Thai links) may do, but it's turned off by default.
If it's the latter,  you'll need to telnet to the router,  and do something with the 'ip nat loopback' command,  but it's a bit vague.

[#4]

OK well the loopback test works and also the canyouseeme site indicates the port is open and not blocked. I'll have to look for the problem elsewhere, thanks for all the help, it's been very useful indeed.