[ VPS ] — [ NET ] —- [ NAT ] — [ UNIFI ACCESSPOINTS ]
If You can connect only one AP at time to newly created site on Your Unifi VM controller, and AP from NAT network connect to VPS f.g.
check if You have:
net.ipv4.tcp_tw_recycle = 1
and revert it to
net.ipv4.tcp_tw_recycle = 0