Connecting to PPPOE ethernet over USB to ethernet adapter I have an ASUS Infinity tablet, running Android 4.1.1. 

I plugged in my USB to Ethernet adapter, and it said that the ethernet port was detected. How do I set up my ISP username and password?
You can try WiFi PPPOE, although meant to be used over Wifi, it might be able to use wired Ethernet as well, depending on how it is implemented (wired Ethernet is not a standard Android thing).