Will a 2A 12VDC PSU harm a 3A OnHub wifi Access point?
All concern scenarios are possible, including that the combination will work. All depends on details of PSU design and TPLINK and PSU design margins. Consider the following:


PSU might have no problem if it is designed to switch to constant current mode when the load reaches the threshold, with proportional drop in voltage. Or it can turn voltage off, and fall into "max short current" mode. You can  research your PSU using variable load (variable resistor, potentiometer) to figure this out.
The TPLINK power consumption (3 A) is likely overstated, with some margin. We don't know what these safe functional margins are.
The PSU capability is likely understated, and with free-air cooling at room temperature it might output some 20-30% more than 2 A. So, overstated consumption with understated limits of PSU might provide working combination under normal room conditions.
If the PSU has the soft voltage cut-off, there might be a possibility of so-called "brown-out" conditions for the TPLINK device. Device protection against the brown-out conditions (exposure to under-voltage transients) requires some special engineering efforts, and might be skipped in quality assurance test program for cheap consumer-grade equipment. So yes, there is some possibility that TplINK hub might suffer from using under-rated PSU. But who knows...


In summary, use your under-rated PSU at your own risk. Or visit your local Goodwill donation center and get a 3-A PSU.