Technicolor TG582n, server hosting with local access

[#1]

Hi,
I'm new here (fibre just installed today) and scratching my head with a problem.
I'm on residential, with one static IP. At the moment I'm using the TG582n as the Draytek 2820vn I was using on ADSL will apparently throttle the fibre speed. Perhaps I need to buy an upgraded Draytek, but as my home networking has simplified a lot over the years (just a single server now rather than a rack of them) I thought it might be possible to get away with using the standard free one.
So, what I want to achieve is:-
External IP address (e.g. 123.123.123.123) is static
DNS for mail.mydomain is set up, and resolves to 123.123.123.123
Internally, the mail/web server has  (say) IP address 192.168.10.11 , and is not in the DMZ (I have tried it both in and out).
I've set up the "game and application sharing" so that the relevants ports are forwarded to 192.168.10.11 (SMTP, IMAP, HTTP etc).
These are all available externally - so mail is flowing in nicely, from a computer outside my network I can connect to my webmail etc. Lovely.
Inside the network I can pick up mail by connecting to 192.168.10.11.
The problem comes with our laptops and phones, which obviously get used both internally and externally. If I'm on the internal LAN, then I cannot connect to the external IP 123.123.123.123 and be routed back in through to the server on 192.168.10.11. If I'm outside it works fine.
I'm obviously missing something on the router setup; this has just worked on the various other routers I've used.
Any ideas, other than replacing the router or setting up internal DNS to work around it?

[#2]

Check if NAT loopback is enabled http://community.plus.net/forum/index.php/topic,110464.0.html/

[#3]

Thanks - that sounded like a winner, but it already was enabled.
I tried clearing and setting it again anyway, but no luck.

[#4]

I just edit my local hosts file (on Windows machines) as follows
Click on Start, type CMD, right click and run as administrator
cd \windows\system32\drivers\etc
notepad hosts
Change settings in for your website then save. Job done
Also good for when you set up a few websites on a server locally and want to see how they run without "releasing them into the wild"

[#5]

That is fine for fixed machines (and I do the same), but for a laptop (or phone) that is used in and out of the LAN several times a day messing with the hosts file gets a bit wearing.
In that case I'd set up an internal DNS (which is handed out by DHCP on my wifi) that maps mail.mydomain to 192.xxxx whilst the real external DNS still has 123.xxxx
It's a kludge though, and would mean having a second DNS server running in the house (the current one acts as a hidden primary for my domains, so I can't just add some zones to that without messing up external access).