I think you've dobe well so far, but at this point it needs the honda shop. There are a few possible root causes here.
Your engine uses a valve system that essentially has two camshfts, one foor low RPM operation and one for high RPM operation. When the ECU detects that the engine is at higher RPMs, it activates a secondary rocker arm system that changes the engine's valvetrain operation fromm the "slow speed cam" [my term] to the "high speed cam". It does this by (via a solenoid) "flipping" a control valve that redirects some pressurized oil to a hydraulic channel that engages the high speed rocker arms. It's using the oil as a hydraulic fluid.
What's gone wrong is that either
the ECU is not sending the signal based on the engine speed
the solenoid is not switching the valve that redirects the oil
the oil is for some reason not activating the highspeed rockers
or perhaps everything would work fine but the oil pressure is blocked from the valve assembly, which would mean the valve is getting switched by the solenoid but there's insufficient oil oressure to do anything.
Sorry, but there's no magic pill, no single likely part. Someone knowledgable will have to do some in depth diagnosis.