One other thing - the old iPhone still works fine as a hotspot. It's just the new phone that stopped working.
I found this discussion that says Hotspot isn't included in our DIGITS plans. The only workaround I can think of is to change the phone plan from DIGITS to a regular plan with it's own number, then you can download the DIGITS app to get your 1st phone number working on it.
Thank you tmo_ian. That explains everything. It doesn't work because that's not included in the plan! Would be interesting for the marketing folks to consider adding the capability at some point - we know it can work technically...