To perform federated backups using NMM, you must create a client resource for the Windows cluster
name from which backup is performed, and dummy client resources for the other participating nodes in
the cluster. Configure the NWR client using the short name of the SQL Virtual Server
By the way is this AAG?

If this is AAG then use Clustername or AG listner for Client name

might wanna shed a bit more light on what you actually have in place? AAG? If so then I'd expect a federated approach?

"If you are backing up a SQL clustered instance, use the virtual SQL Server name here. Create client resources for all the physical cluster nodes where the SQL clustered instance is being run. "

https://www.dell.com/support/manuals/en-us/networker-module-for-microsoft/nw_p_nmm_sql_sharepoint_vss_guide/performing-federated-backups-on-sql-setups-that-have-alwayson-availability-group-configured?guid=guid-cba1eac4-d5d4-48cf-835e-b75a31d76b90&lang=en-us

BTW it looks to me that some of the examples given in above link (also for nw19.8 and 19.9 online guides) incorrectly state "%5C" instead of mentioning just a backslash in the saveset name.

"nsrnmmsv -P" should show the databases that you'd might wanna specify to be backed up in the AAG.

install  client , extended client and NMM in both nodes > make sure to install /run them with a service account that has permission on DB  (if u are installing with normal user nsrexcd must run with the service account with permission to DB )> in NMC first config node A & B take full backup of DB  > then config the 3rd backup with cluster fqdn name : run full backup - make sure the permission are given in both node and cluster and all the DB 

it will need > backup operator > admin & remote desktop user permission