Question about Synology RT2600ac

Generally speaking, the 2.4 ghz frequency is more far-reaching than 5 ghz. With 5 ghz, I would say that you can still barely cover a floor below or above (vertical coverage) through a floor that is about 30 cm thick. In horizontal coverage, I have found (with 5 ghz) that there is clean coverage from my Redui (through the kitchen) to the end of the living room (but this is because of how the antennas are oriented). For the best signal propagation, you should align the antennas horizontally at a 90 degree angle (see description in the attached manual).

PS: about the kitchen --> I don't have a microwave that could interfere with the 2.4 in operation.