Dynalink RTA100 is a pig of a router.
I cannot remember what configuration options they have anymore.
From what I gather you have the RTA100 at the workshop where your server is
If this is correct, do you have the computer at the workshop on a static IP?
If you do, what software are you using to remotely connect to said remote computer?
Depending on what software you have chosen, have you setup the relevant port forwards on the RTA100 to point to the remote computer?
PM me if you are completely stuck.