Di2 gears not displaying

Any bright Di2 souls out there?

Di2 only displays Gear Combo 0-0 on Garmin Headset. 

So far:

Tried a different Garmin but displays the same 0-0

Swapped out Di2 dongle from my other bike and dongle works on the other bike showing gears on the other bike correctly,  so have excluded the dongle as the cause (?)

All gear changes work fine so have (?) excluded the wires from the dongle/junction box to front and rear mech and to hoods. 

I’m left with it being the short connecting wire from the dongle to junction box, or the triple junction box.
Does that sound about right to you? Any other ideas?

Short wire would be the cheaper of the options to buy but still £22   :0  Unnecessarily changing lots of Di2 parts would be ££ even if I could get hold of them currently.

[  Before you ask why I’m chasing this, Its Di2 on a Tandem, 50:34 , 11:42 so we’re running a necessarily too short a chain for this range. I need to know what gear we’re in so as not to break the rear mech by accidentally getting even close to crosschaining. (again anyway, that was an expensive error a couple of years ago) ]
 

Grateful for hive wisdom.