You'll have to be careful here. It sounds like your second router is meant to link your home network to your modem. If so that means it has network address translation capability and will act as a DHCP host, assigning IP addresses to the devices on your network.

In the context you are attempting to use it this is utterly unacceptable, the results will be erratic and hard to troubleshoot. You must never have two DHCP devices on your network and having two NAT devices on your network is something for experts only.

If you can turn off these capabilities and simply use it as a pure router what you are trying to do should work. I have no idea whether your particular unit can. Unfortunately, there's a lot less demand for simple bridges so they tend to cost more than routers that have every bit of their capability.