L2TP Port 1701 'filtered/closed' and unable to open!

[#1]

Slight problem with a TCP port - 1701, for use with L2TP. Tying to set up a VPN using Server 2016 Remote Access feature. It's not going well. 

Quite literally, I'm finding it impossible to open the above port. Plusnet firewall is OFF, I've tried with both my pfSense router and a Netgear WNDR4000. Both have the necessary options to allow the port to be free but nope. Nothing. Interestingly, forwarding the Microsoft AD port (445) all works fine. Along with 25 (mail) etc etc. Checking on yougotsignal shows it firmly closed. 

Has Plusnet recently blocked this port off? 

[#2]

I don't believe that PN block any ports if the Firewall is set to OFF. However, was it already off or did you have to change it to OFF ? Any change is firewall setting will not take effect until you drop & re-establish the PPPoE connection.

[#3]

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@FoolishlyWise wrote:

Slight problem with a TCP port - 1701, for use with L2TP. Tying to set up a VPN using Server 2016 Remote Access feature. It's not going well. 

Quite literally, I'm finding it impossible to open the above port.

You are absolutely sure the problem is a blocked port?

L2TP requires AH and ESP to traverse the firewall; IP protocol #50 and #51 respectively.  Note: protocol numbers, nothing to do with port forwarding.

[#4]

Seems like it - forwarding port 1701 (even if it was for use for another application or anything) shows it closed, even when setting it up using the most simple router or by pfSense. Haven't got onto setting IPP yet - was trying to get TCP 1701 and UDP 500 working first (the latter seems fine). 

[#5]

Are you sure that there's a server running on the target machine port 1701 ?
If not then the test at "yougotsignal"  may not be reliable.

Downloading the port checking tool at portforward.com is a more reliable test.
https://portforward.com/help/portcheck.htm

[#6]

Downloading the port checking tool at portforward.com is a more reliable test.

Or, if you use a proper operating system [sic]

# nc -l 1701

[#7]

Thanks, never though of using netcat for this.

nc -l -p 1701

Works well on my windows 10 machine