Fyi. I bought a TP-Link Archer T9E and despite it's out of the box status it did not work OOTB. The card was recognised, but it showed only a few wifi networks and trying to connect to one failed after typing in the correct password.
After googling i found people have been having problems with it in a specific PCIe slot which in the case of my HP 8300 MT seems to be the x1 slot for which it is mainly designed. So i moved it to the x16 slot and now it's working without issues! You could also move it to the x4 slot, but i couldn't because the heat sink of my Asus 710-2-SL graphics card is blocking it.
As a side note, I had to put the gpu to the x4 slot because when it's in the x16 slot the heatsink blocks the x4, leaving the x1 as the only free and accessible slot in which the wifi card doesn't work properly. The gpu is an x16 card, but since i'm not doing anything graphic intensive it doesn't matter to me and it works just as before.
If by any chance anyone has any further info on this matter, shoot up. It would be of course optimal to be able to put it in the x1 slot!