Home network issues

.My setup:I have a router from my ISP (VOO Belgium).From this router, I connect to a 10-port switch (5 PoE + 5 standard).From this switch, I connect to two Zyxel NWA50AX access points (1 on the ground floor + 1 upstairs).The other wired connection points are:- in my living room, the TV box- a desktop PC running Windows 10;Cat.6 UTP cable throughout the house.The devices easily find the Wi-Fi network in the house, no issues, but internet access is difficult for some devices, particularly occasionally for the iPhone (I have to switch back to 5G to open a page because I see it's not downloading); the tablet too... Without 5G, I have to disable and then re-enable the Wi-Fi on the tablet to regain internet access.I have a connected doorbell, but it has trouble making an outside call... Unless I'm using my ISP's router's Wi-Fi. Then it works fine...I haven't done the same test with my AVR (my connected amplifier), but it also easily loses internet access, and I then have to reboot...That's the problem: the internet access isn't stable when I'm using access points, but also via a wired connection.I also tested it with professional PCs that use secure connections: no problem connecting to the ISP's router's Wi-Fi, but it's extremely difficult (and sometimes impossible) to access the internet via access points...I don't understand what could be blocking internet access...Is it possible that this could be due to the cable connecting the router to the switch?