Multiple Router Learning

Hello, as a hobby I like to learn what I can with various tech out there, my current project is learning how to connect 2 tp-link routers essentially in two different locations. Currently I can only have one connected to the modem to receive a public ip address. I was wondering if it is possible to have a second one connected with a separate public ip address to work on this. Having one connected to the other so far is causing loop back issues and hoping to have use of a second public iPhone address to prevent the loop back. Any and all information is greatly appreciated.