I'd get rid of the Cisco then, unless it does something special for you.
The IP that is being given out by the cable modem can easily be given to the MT router with default gateway and everything you need for internet access.
Simply connect the MT to the modem on its own and go to Winbox and do the following.
Click IP > DHCP Client > click + > add DHCP client to the port that you plugged the cable modem in to.
Click IP > Address > click + > Add IP 10.0.0.1/24 (include the /24) and choose a port on the MT to assign the IP to. (this is the port going to your LAN switch)
Click IP > DHCP server > Click DHCP server setup and follow instructions.
Click IP > Firewall > NAT > Add src-nat rule > set out interface to ether port that the cable modem is on > set source IP to 10.0.0.0/24 > set action to masquerade
Click PPP > enable the PPTP server > Click secrets and Add a secret (secret = logon account) > set remote IP to something like 10.1.0.1 and local IP to 10.0.0.1 also (leave out the /24 here)
try connect to your public IP from a windows computer, use the logon and password that you created in the secret tab above.
Read the MT wiki about firewalls and vpns for extra info.
Sell the cisco and buy some beer with the money. consume the beer while working from home over the VPN.