how can I use this old router??

Solution
It would work fine.

1) Plug the phone line into the left-rear input.

2) Connect the PC via an Ethernet cable to any one of the other connections.

3) Follow the setup procedure in the manual.

I'm not sure why the first guy says it won't work.
It would work fine.

1) Plug the phone line into the left-rear input.

2) Connect the PC via an Ethernet cable to any one of the other connections.

3) Follow the setup procedure in the manual.

I'm not sure why the first guy says it won't work.
 
Solution
Most places where the internet connection is an RJ45 plate in the wall use ethernet back to a basement switch. Not always; standard practice here in NZ for new homes is DSL in an RJ45 port with master splitter, but that's still rare.

I've seen many threads out of India where that's the case.