I've been having trouble adding new devices to my R7800 router, but I think I solved the problem for me. You can jump to that down at the bottom if you don't want to read the entire post.
Firmware V1.0.2.82
I use access control and block all new devices from connecting. I just bought a Samsung S21 and I could not "Allow" it to connect using the Advanced/Security/Access Control settings. I tried selecting the box next to the S21, then allow, then refresh and it would always indicate that the S21 was blocked. Then I tried to select the box next to the S21, then edit, then selecting allow, apply, and back to the main Access Control screen where it said the S21 was still blocked. I checked for router firmware updates and none are available.
I noticed that the S21 had a setting to use a "randomized MAC address" so I deselected that to use the phone's MAC address in case that was causing issues, but that did not solve the problem. The S21 would connect if I chose to allow all new devices to connect and then it showed as allowed in the devices list. It would change to blocked if I then chose to block all new devices from connecting. I don't know how many times I tried reconnecting my S21, forgetting the network, logging into and out of the router, all to no avail.
What worked for me:
I turned off the S21's WiFi.
I selected allow all new devices to connect on the router.
I turned on the S21's WiFi, entered the network password and made sure I was connected and that I could access the internet.
I rebooted the router through the Advanced home tab under Router Information there's a reboot softkey.
I logged back in to the router after the reboot and verified that the S21 was still connected.
I selected block all new devices.
The S21 stayed "allowed" and I'm up and running.
I am using the phone's MAC address and will probably turn on the Randomized option when/if I connect to a public network.
