why is it i cannot connect to the internet using the PLDT router when i connect it to the Desktop computer?

It could be:
IP address conflict
Operating System Firewall
Operating System Network Card drivers
You are connected to the wrong port on this Router
Router security settings may require you to register your Network Card MAC (address) on the Router first.
If using Wifi, you may be using an incorrect password
If using Wifi you could be trying to connect to a neighbours Router and not your own.

There are more things we could write here, we would need to know a lot more about your computer, and your Router.