I am about to upgrade one of our clients domain and forest functional levels from 2008R2 to 2016. They have a mix of 2016 and 2019 servers in the forest. They mentioned they have a RADIUS server. Do I need to be concerned about that or just make sure that DC's are compatible/compliant to the target level? I have done AD health checks and replication checks, but wasn't sure if I missed anything. Also, what checks need to be done after proceeding with the upgrade? Any insight would be appreciated.