No one knows why, but Roku has two slightly different numbers that it refers to as serial numbers in different places. The number that's displayed as a linked device at https://my.roku.com should match the number displayed under Settings->System->About on the Roku device, but not the number on the device itself. Yours was probably linked. Roku had some kind of outage today, so keep trying.
Warning: You may be disappointed in what's available on Roku in the Philippines since that's not a country Roku supports.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee.