Spark hg659b - Can't Forward port 1701

Hi there

I am trying to set up a VPN server on my Synology NAS using L2TP/IPSec. Apparently to get this to work I need to forward ports 1701, 500, and 4500 (UDP)

The HG659B refuses to allow me to forward port 1701. "The external port cannot be 7547,514,68,53,546,500,1701,8443,8080,990,5060,28090,50000-50020."

(Note the error message also mentions 500, but this can be forwarded with a built-in mapping).

Couple of questions:

1) Any idea why?

2) Any way around it?

There are no options to change the ports being used in the VPN Server settings.

Thanks!

Brett