951G - wan port forward

Is it possible (I have no doubt it is) to configure the 951 so that 1 lan port so this is a 2nd “wan” port ?

My cable provider’s settopbox needs an external IP address in a completely diffirent subnet to make it work as expected. (settopbox is recognized by it’s mac addres and therefore is put on a settopbox only ip range.)
I have the modem connected to port 1, and port 5 should also get an external IP address provided from the ISP.

On my old DD-WRT I solved this by putting the WAN port an a LAN port in the same vlan. This way devices on this port also received a WAN IP.

Picture:
https://www.dropbox.com/s/yg7ct9qzqm6mej6/951G-2HnD.jpg?dl=0

I’ve managed to get things to work.
Is this a good, secure config ? Any suggestions ?

/interface bridge
add name=bridge-digibox
add admin-mac=D4:CA:6D:9A:39:F1 auto-mac=no name=bridge-local
/interface wireless
set [ find default-name=wlan1 ] band=2ghz-b/g/n channel-width=20/40mhz-ht-above country=belgium disabled=no distance=indoors \
    frequency=2452 ht-rxchains=0 ht-txchains=0 l2mtu=2290 mode=ap-bridge ssid=xxxxx wireless-protocol=802.11
/interface ethernet
set [ find default-name=ether1 ] name=ether1-gateway
set [ find default-name=ether2 ] name=ether2-master-local
set [ find default-name=ether3 ] master-port=ether2-master-local name=ether3-slave-local
set [ find default-name=ether4 ] master-port=ether2-master-local name=ether4-slave-local
set [ find default-name=ether5 ] name=ether5-settopbox
/ip neighbor discovery
set ether1-gateway discover=no
/interface wireless security-profiles
set [ find default=yes ] authentication-types=wpa-psk,wpa2-psk mode=dynamic-keys wpa-pre-shared-key=d6nw5v1x2pc7st9m \
    wpa2-pre-shared-key=xxxxxxxxxxxxxxxx
/ip pool
add name=dhcp ranges=192.168.100.10-192.168.100.20
/ip dhcp-server
add address-pool=dhcp disabled=no interface=bridge-local name=default
/system logging action
set 2 remember=yes
/interface bridge port
add bridge=bridge-local interface=ether2-master-local
add bridge=bridge-local interface=wlan1
add bridge=bridge-digibox interface=ether1-gateway
add bridge=bridge-digibox interface=ether5-settopbox
/ip address
add address=192.168.100.254/24 comment="default configuration" interface=ether2-master-local network=192.168.100.0
/ip cloud
set enabled=yes
/ip dhcp-client
add comment="default configuration" dhcp-options=hostname,clientid disabled=no interface=bridge-digibox
/ip dhcp-server network
add address=192.168.100.0/24 comment="default configuration" dns-server=192.168.100.254,195.130.131.130 gateway=192.168.100.254 \
    netmask=24
/ip dns
set allow-remote-requests=yes
/ip firewall filter
add action=drop chain=input comment="default configuration" protocol=icmp
add chain=input comment="default configuration" connection-state=established
add chain=input comment="default configuration" connection-state=related
add action=drop chain=input comment="default configuration" in-interface=bridge-digibox
add chain=forward comment="default configuration" connection-state=established
add chain=forward comment="default configuration" connection-state=related
add action=drop chain=forward comment="default configuration" connection-state=invalid
/ip firewall nat
add action=masquerade chain=srcnat comment="default configuration" out-interface=bridge-digibox
add action=dst-nat chain=dstnat dst-port=4500 in-interface=bridge-digibox protocol=tcp to-addresses=192.168.100.100 to-ports=4500
/ip firewall service-port
set ftp disabled=yes
set tftp disabled=yes
set irc disabled=yes
set h323 disabled=yes
set sip disabled=yes
set pptp disabled=yes
/ip service
set telnet disabled=yes
set ftp disabled=yes
/ip upnp
set allow-disable-external-interface=no
/snmp
set trap-community=public
/system leds
set 0 interface=wlan1
/system ntp client
set enabled=yes primary-ntp=157.234.197.5 secondary-ntp=157.193.40.37
/system routerboard settings
set silent-boot=yes
/tool mac-server
set [ find default=yes ] disabled=yes
add interface=ether2-master-local
add interface=ether3-slave-local
add interface=ether4-slave-local
add interface=ether5-settopbox
add interface=wlan1
add interface=bridge-local
/tool mac-server mac-winbox
set [ find default=yes ] disabled=yes
add interface=ether2-master-local
add interface=ether3-slave-local
add interface=ether4-slave-local
add interface=ether5-settopbox
add interface=wlan1
add interface=bridge-local

May i ask why digibox on port 4500?
Is this digibox from telenet?

Maby can help my website by other topics
http://www.wirelessinfo.be/index.php/mikrotik/pages/menu