1. Check connectivity of cables (better if you have a LAN tester), nowadays patch cords are all straight not unlike bef0re when you used crossed.
2. Check the current mode of your PLDT modem/router (yes even the old modems of PLDT can act as a router). Recommended setting is
bridged m0de and not routing mode...check for the default local ip address of your PLDT networking device and access its web configuration.
3. When you connect your modem directly to PC, check what kind of IP does it get...(could be a public or a private IP assigned either static/dynamic)

Naa lng ni gamay nga misconfiguration bai...most likely, unless there's a problem with the WAN port of your router.