Can I connect my DSL cable directly into my WiFi router?

So I have a SpeedStream DSL router that was provided by my internet company (AT&T I believe) to stream DSL, a couple of years ago we got a Cisco Linksys WRT54G2 V1 wireless router to hook it up to so that I could have WiFi for the laptop I had just purchased back then. The router is fairly basic, having an "Internet" port (taken by the cable from the wired router) and 4 "Ethernet ports" that are unused, the DSL router from my internet company has an "Internet" and "DSL" port which are both taken. Although the "DSL" cable doesn't connect to my router currently, could I put it into one of the Ethernet ports on the WiFi router to save space and get rid of the DSL routers? Or would it not work? Also, due to having two devices registering the internet, could the speed be bottlenecked? I feel like it's pretty slow even though my phone says it's a 50 Mbps connection (although I get about 2.5 Mbps downloading and .41 Mbps uploading when I tested it)