How to set up DVR? Problem with port forwarding with router used for cash register

I installed DVR and connected it to network but here is my problem.
Location where this is installed is franchise and router is used for cash register. Router is protected with username and password and I can not log in to do port forwarding and francise owner don't want them to know that he installed cameras. I didn't know about this when I started work. So network there is: DSL MODEM AND LINKSYS BEFSR41 ROUTER. I don't want to reset settings on that router, not sure how it was set up for cash register.
Is there any way to do it with another router or router switch combination
I just need to forward ports 5000 and 5001 to my DVR.
IP is static at location.
Any help would be appreciated.

Thank you