Hi,
Kindly note that MS only provide RDC for mac with latest version in the official link and there is no other way to find previous version rather than two options:
1.remote desktop service for mac beta: it not only provides with new features than official released version but the link below also stores some previous versions back to Sep 05, 2019 (earliest)
https://install.appcenter.ms/orgs/rdmacios-k2vy/apps/microsoft-remote-desktop-for-mac/distribution_groups/all-users-of-microsoft-remote-desktop-for-mac
2.previous versions of RDC provided in third party websites which may have security risks.
I searched a while and found the OS version you mentioned is released on 2017. If the beta version did not work, you'd better seek advice in the support forum of Apple.
Hope this helps and please help to accept as Answer if the response is useful.
Thanks,
Jenny