I just changed ISP. The new one is much cheaper, but they don't provide a router or modem, which ever it is.

The guy just put an internet cable plug on a cable in my Telecom wall box, went downstairs, did something, and bam, I was connected to China Mobile.

I did not have router. He said get a TP-Link. But first, I just plugged the cable straight into my laptop. I couldn't connect. So I set up a dsl connection in Ubuntu, using my phone number and a password they gave me. Connected ok.

Then I bought a TP-Link 450MBs router. My laptop could not connect via dsl through the router, but it did connect, after setting up the router through 192.168.1.1, through ethernet and wireless.

My question is: What is the difference between dsl and ethernet??

ethernet is a kind of a network, and you can simply connect to it. dsl is used connect to a service provided by your ISP. and if you are connected you will be able to use that network (provided by your ISP).

Digital Subscriber Line is supplied by a vendor to allow you to connect to the internet.
Ethernet is the physical connection (by wire) between 2 or more computers.