Back to My Mac issues.

[iFoggy]

Hi,

Have the following config:
- 'Dumb' fibre CPE
- Airport TimeCapsule latest generation, latest FW
* DHCP & NAT
* Enabled NAT port mapping
* Defined static IP address of Mac mini as default host
- Mac mini 10.12.14 (iCloud Back to My Mac service running)
* Firewall closed but with SMB allowing incoming connections - only have SMB as a File share
- MacBook Pro 10.12.14 (iCloud Back to My Mac service running)

All of the above works perfectly except I cannot see the FS on the Mac mini remotely via the MacBook Pro

Following works: (which proves I can access from the outside)
- Plex media server running on the Mac mini works remotely
- Can do screen sharing to the Mac mini
- Can ping the default host IP which is routed to the Mac mini when I allow ping on the FW on the Mini, disable ping and that fails so means I can see the FW
- If I enable 'Back to my Mac' on the Airport can see the TimeCapsule disk, after that remove back to my Mac
- Can see the Mac mini in the 'back to my Mac' section of Finder but just get a 'connection failed' when waiting to see file share
- It would just saying 'connecting' for about 2 - 3 minutes then say 'connection failed'

Tried the following:
- Reloading Mac mini
- Turning FW on / off
- Tried SMB / AFP then them separately
- Read every article I can find on line.
- Deleted the passwords for Back to My Mac in keychain

So pls help if anyone has any other suggestions?