Galaxy S9+ won't power on after motherboard replacement

Hello.

I replaced the motherboard on my Samsung Galaxy S9+.

Current symptoms:

• The phone charges normally and shows the battery percentage.

• The backlight turns on when I press the volume button while charging.

• However, the phone will not power on.

• The power button does not seem to respond.

I tried:

• Holding Power + Volume Down for 10 seconds

• Holding Power + Bixby + Volume Down

• Reseating the motherboard

• Tightening all screws

The phone clearly has power (charging works and backlight responds), but it will not boot.

Could this be a power button contact issue or something related to the frame contacts?

Any advice would be appreciated.