Hi John,
I can imagine it’s a challenge. To me I read this as they are probably more end users than experts. But in either case you need their know-how to be able to get the testing done.
One suggestion could be to set up some recurring virtual touch point, a digital demo meeting - or a simple phone call. If visits are totally out of the question.
Another suggestion could be to designate a Proxy SME in the team, that similar to a product owner is the person to go to. In the team this could either be the tester or ux’er, if available.
Futher reading - the recent power hour on SME’s:
Read more on wrt domain experts and SME’s here