@SlySly 

Here's a guide for configuring Address Reservation on your router.

If your DHCP server IP address pool is from 192.168.1.100 to 192.168.1.249 (for example), make sure that IP addresses that you set in the Address Reservation table are within the same range.

Also beware that some devices like mobiles and laptops could use a random MAC addresses each time when they are connected to your wireless SSID.

Here's how to mitigate this behaviour.

  @SlySly 

So the IP address that devices received from AX23 are within its DHCP server IP pool, but are not the ones you've set into the Address Reservation table ?

Are these IP addresses (set into the Address Reservation table) within the DHCP server IP pool as well ?

Your router is Archer AX23 HW v1.2 - is that correct ? - you can check that on the device bottom sticker.

Was the Address Reservation working normally before ? Have you done any FW upgrade recently ?

Can you describe your network setup - any ISP provided devices or other network equipment, etc.? It would be easier to troubleshoot if you post a screenshots of your DHCP server settings, Addrres Reservation table and DHCP Client List table. You can cover/blure your MAC addresses first three hexadecimal pairs for privacy.

  @SlySly 

How many devices you've addedd to the AX23 Address Reservation table ?

I can see where the problem is with one of your PCs (Sly-dell-2-Pc) - it has two address reservations under the same name.

That's from Address Reservation Table:

That's from the DHCP Client List:

Another entry for Sly-dell-2-Pc with an issue.

From Address Reservation table:

From DHCP Client List table:

When Sly-dell-2-Pc address reservation is enabled/active, device should appear in the DHCP Client List table as "Permanent" but not with its Lease time as in the screenshot above.

Lets take for example Lenovo-Pc - that's how it should look like to be correct.

That's from Address Reservation Table:

That's from DHCP Client List table:

You can see the difference between those two devices.

  @SlySly 

Hi, except for the router, do you have any other access point or range extender in the same network?

If so, try connecting Sly-dell-2-Pc etc. to the router directly, also try to remove the address reservation for Sly-dell-2-Pc and add it back again to see if it will work.