@ku4eto 

Hi,

are your routers US versions or European versions (code EU)?

Especially if they are US versions, with the AX10 being TP-Link's entry-level Wi-Fi 6 offering it is entirely possible that the transmit power has been limited to much less than the 1000 mW (30dBm) that are permitted by the FCC regulations in the 2.4 GHz band. This could be due to the hardware not being able to output the full 1000 mW or for other reasons.

@woozle Its the EU version tho.

@ku4eto 

Yeah, to output the meager 100 mW (20 dBm) that the EU rules permitt for the 2.4 GHz band should be no problem for any Wi-Fi router model.

I am no expert on this, but I wonder if the "Beamforming" feature of the AX10 could be the culprit in this case? The C50 apparently doesn't have Beamforming technology and if its antennas are adjusted vertically it should therefore emit the permitted 100 mW horizontally in any direction (usually in a donut-shaped radiation pattern). The AX10 with Beamforming should only emit about 100 mW in the direction of the radiation "beam" and if for some reason steering the direction of the beam wasn't working well in conjunction with your Xperia Z5, then maybe it could result in what you are observing?

Anyway, I am just guessing and speculating here. 

@woozle Well, the antenas are in the same direction, the router is literally on the same place as before. Antennas are slightly bent, at about 10-15* (not completely vertical).

I know that the waves are not  getting produced from the tip (going vertical), but still, its a wave, that propagates. Just seems that the signal is too weak for the 2.4Ghz, especially considering, that the router should actually be more powerful.
And  2.4 Ghz should be using less electricity from 5Ghz (presumably, we can say its not power limitted) and the wave frequency should allow it to propagate further (and deal better with obstacles, by not getting absorbed, due to the wavelenght).

But you say, its entirely possible to be limited by standards regarding electronic interferrence (or whatever it was called here in EU). That would mean, a FW update would be necessary, to adjust the power distribution.