This is DST NAT chain in firewall.
You need to forward whatever protocol / port to the specific device inside your network.
Default Port is 1723 for built in VPN.
Lets say you have your VPN server sitting at 192.168.1.99. And your routers WAN connection is ether-1
/ip firewall filter
add action=dst-nat chain=dstnat disabled=yes in-interface=ether1 protocol=gre \
to-addresses=192.168.1.99
add action=dst-nat chain=dstnat disabled=yes dst-port=1723 in-interface=ether1 \
protocol=tcp to-addresses=192.168.1.99
Open new terminal and put that in.
It will add 2 rules to the dst-nat filter.
Winbox
IP
Firewall
NAT
They are both disabled.
Locate the rules and change the specifics like IP address of your VPN server and interface for your wan connection. Then enable rules.