Screen sharing via Back to my Mac

I'm away from home for work.

I have my iMac at home, connected to my internet connection which I know is working. It is on the DMZ on my network so there are typically no issues with blocked ports and the firewall is turned off.

With me I have my Macbook Pro. Both machines are on 10.10.2

When I sign into BtmM on both machines the iMac shows up in finder on the MBP, however Finder says not connected and when i click share screen I get connection failed.

What can I do to troubleshoot this to try and find what part of the chain the issue lies in? I won't be home for another few weeks, but should be able to make changes remotely (the iMac is on, I'm using it to chat to my partner daily and managed to connect remotely via Team Viewer tonight)

TIA