@DavidAbin 

From which menu of the router did you remove this wireless client ?

If its from the Advanced => Network => DHCP Server => Address Reservation you can add it again.

If it is from Network Map => Clients => Connected clients you can remove it from Access Control Blacklist / add it to Access Control Whitelist.

  @DavidAbin 

Here's the Access Control menu:

This feature is available only when the device is in wireless router mode although some devices offer Deny list in AP mode.

What's important here as I posted is to know from which router menu you deleted the device in question.

  @DavidAbin 

Just to make sure that we're on the same page can you post a screenshot from the router's menu where you've deleted this device.

If you add a device to your Deny list it means this device would not be able to connect to the router and to the internet.

The mac address of a device can be seen on the router itself when that device is connected to the router:

  @DavidAbin 

From these screenshots I would conclude that you have deleted a device/devices from your Access Control Allow list.

Addig their MAC addresses to the Allow list will allow them to connect to the router and to the internet.

Here's how to check your computer MAC address.

Here's how to check your router MAC address.

Here's how to check your phone MAC address.

For other devices you can look for a sticker/label on the device itself.

If you can't find a device MAC address but you need to add it in your Allow list do the following:

Disable Access Control on the router. Connect the device to your router - check/copy its MAC addres on the router like I posted earlier.

Then Enable Access Control again and add the device to your Allow list.