Sorry but with windows it is really not possible.. The TC does not open SMB protocol to the internet and the internet blocks SMB anyway.. it is to risky.
The only way to do this.. safely is using a vpn router and relegate the TC to bridge role.
Ok thanks.. Meaning if I get a Dlink or TPlink router, it should work?
Meaning if you buy a router with vpn end points built in, you can access the TC safely over vpn..
Not a standard dlink or tp-link actually.. something decent. VPN is not easy to set up and is worth buying decent quality equipment with plenty of help from the manufacturer on getting vpn to work. So Draytek, Billion here are ok, some of the better end Netgear or Linksys Cisco. Dlink are more the dinky toy end. TP-Link can work but you won't get any help.
You could buy a Mac and use AFP.. BTMM with iCloud.. but I am just pulling your chain.. Apple do not design their stuff with more than a basic nod to the rest of the world. It is a closed environment.. but if you work inside it, everything just works.. cough gollum..gollum.
If you hunt around there are ways of passing SMB (ie windows networking) over the internet without using vpn.. operating on unusual ports.. but the fact is, security is still compromised. If your office has any decent sort of protection on their network, they should have it blocked. But vpn might be allowed.. you need to talk to office IT admin if you work in a place with more than 3 people, to see what their policies are.