Unless the server config is changed, you can do nothing ...
Also,
The Manual wrotep2p -- Use a point-to-point topology where the remote endpoint of the client's tun interface always points to the local endpoint of the server's tun interface. This mode allocates a single IP address per connecting client. Only use when none of the connecting clients are Windows systems. This mode is functionally equivalent to the --ifconfig-pool-linear directive which is available in OpenVPN 2.0 and is now deprecated.
I suggest your server starts to use --
topology subnet