@mohammed394 Reset the AP to ist factory defaults by pressing the reset button for about 10 seconds. Then use this guide for configuring the AP in RE mode with your new router.

@Devonlad That seems to be the normal way for cabling your premises. Just make sure you've chosen the right switch for you in terms of: 1) number of ports 2) at least gigabit port connectivity

@BanjoWest1956 Deco ports are auto-sense so it shouldn't matter to which port of the main Deco unit you've connected the switch. This document could shed some light on the topology that you should...

@Hozzu Neither can I - I just saw this implementation (HTTPS setting only). To mitigate this you can use the Tether app for remote management as well.

@Hozzu Apparently they decided to bet on HTTPS only connection as more secure for router remote management. Did you try to disable remote management and test if there's any difference ?

@AdamK81 This sounds reasonable too and you can try that as well.