Mounting external NFS share - what am I doing wrong?

Hi, community.
Stupid problem here.
What I am trying to do:
I’d like to mount external NFS4 share to my HA and store backups there.

What am I doing:

  1. I went into HA Settings → System → Storage, but it said “The operating system does not support network storage”, complains it needs OS 10.2. I am running HAOS in Proxmox, all updates are installed and I even tried to install DEV. I am running supervisor 2023.7.2, OS 11.0.dev20230501 and front-end 20230705.1 - latest. No further updates available.
  2. I enabled SSH, logged in into HA and tried to run mount command manually - mount -t nfs4 -o nolock backuppi.house:/nfs /media/backup/. The command ends up with error about access denied by server, but when I am running the same mount command from my workstation or from another server on the same subnet, all works OK and everything is mounted/unmounted no problem.

Am I missing something?
Can you, plz, suggest how can I get the HA to offer Network storage? In the end, I do run OS version 11 and it needs only 10.2.
Thanks for your suggestions!