Welcome to the fun, i’ve been having to deal with FSP’s in regards to some ad migration project.
the first thing to know, is you can only view/touch FSP as objects, and the native get-adgroup cmdlets fail spectacularly as soon as an FSP is involved.
second, memberof is not avaialble on adobjects.
I haven’t had to solve for this specific deal, some others have some ideas here:
https://www.reddit.com/r/PowerShell/comments/59ofw3/find_users_group_membership_in_trusted_domain/
alternatively you can try with raw ADSI, or try with adsips, however that will fail when you have orphaned sids unfortunately.
https://github.com/lazywinadmin/AdsiPS
