mounting CIFS share (tcp/455) with FreeBSD and mount_smbfs(8)

I need to mount a CIFS share from windows server 2012 r2 via CIFS, tcp/445 as NetBIOS
service (tcp/139) has been deprecated due to serious vulnerability issues. 

Until the disabling of NetBIOS and tcp/139 we used successfully autofs and mount_smbfs.
this is no longer working. I tried to force autofs/mount_smbfs to bind to port 445 on the
server via ://_at_xxx.xxx.xxx.xxx:445/sharename, but this doesn't work.

Trying to mount a share from a samba 4.3 server (FreeBSD CURRENT, net/samba43, both most
recent sources), where I configured samba_server via smb ports = 445 to use port tcp 445
only and only SMB2 and SMB3 (server min protocol = SMB2) protocols via the following
command:

mount_smbfs -I xxx.xxx.xxx.xxx -U a_user -W \
WORKGROUP //a_user_at_xxx.xxx.xxx.xxx:445/sharename /mnt

results in the error

mount_smbfs: unable to open connection: syserr = RPC struct is bad

Setting "smb ports = 139,445" and "server min protocol = NT1" seems to work, the share
can be bound, but this is SMB over tcp/139 and not CIFS.

I desperately need CIFS and I need tcp/445 since tcp/139 is from now on firewalled. 

So: what do I miss here?

Kind regards and thank you in advance,

O. Hartmann