It seems that while the phone can use data from either SIM The hotspot can only connect to the internet via the SIM in slot 1.
Not a problem now I have discovered this but I thought I would flag this to save others the same problems I had.

It would be good if an error message was displayed if the hotspot is activated while data through SIM 2 is selected.

Hi there!
Just tried that on my BV8000 Pro in dual SIM set up, and the hotspot uses the mobile connection of the SIM flagged as "preferred for networking" under SIM settings.
I can use both of them....

I went through all the usual resets and then swapped the SIMs before hotspot worked.
Clearly, performance may vary in this respect, it may even only apply to some carriers, just another factor to be aware of.
Using Build V0.1_20170803
Since other than this It's working I intend to wait for Android 8 before updating.