Draytek 2820n - problem remoting in if both host and destination use PlusNet

[#1]

All started with changing to new router.  Sounds weird - but here we are:
Previously  Draytek 2600, connected to SBS2003 Premium (w/ISA 2004) and could remote in from home no probs - use RWW and OWA on SBS
I have a static IP from Plusnet at home (so, x.y.z.(a number) as does the work server (so, x.y.z.(a different number) - i.e. the same IP apart from last wotsit.  Same subnet mask also, btw.

Situation now with newly purchased 2820n (on latest firmware) at work is:
From any PC connected to my home network (which connects to internet via Plusnet) :
- if I try to RWW to SBS, I get a delay in IE then 'IE cannot display the webpage'
- if I try to use OWA in SBS, ditto
- if I try to remote manage the router (http://a.b.c.d:8010 - have altered from port 80), ditto

In an effort to test/solve, have: changed my router at home (from Netgear to Billion)and have tried laptops with different AV systems - no joy.
HOWEVER.... if I slap a T-Mobi 3G dongle onto one of these laptops, and connect to internet that way ... everthing works fine:
RWW works fine immediately
OWA ditto
Remote mgmt of router ditto
***The one consistent factor here is Plusnet (or, rather, using static ips in same range) - is it at all possible that the 2820n has some weird issue with this IP situation at either end?? i.e. it is interpreting it as a 'threat' or 'not permitted' ?? ***
(BTW Just swapped back to the 2600 old router... and problem gone away.  So, definitely a 2820n + PlusNet/same IP set thing)

Thanks

[#2]

Loopback.
Your are the wrong side of the NAT to use the modem external IP address. You need to enable loopback on your modem/router if there is an option to do this. On some you can only do it via telnet.

[#3]

?? confused by your answer.
Have you taken into account the fact that
a) I could remote in fine (to both the router management console AND to Small Business Server 2003 features for 3 years until I changed the Router
b) I can still remote in to both... provided I don't use Plusnet at my end
... if so, you might need to explain "Your are the wrong side of the NAT to use the modem external IP address" - as I have been using the router/modem external address for all that time (on the new Draytek I just entered the same settings as on the old, i.e. gave it the Plusnet IP address as my 2nd Routing IP).
'Loopback' - I'll have to look up the definittion of this, as I am unsure what it means in a router context.