RAX20 remove connected device from memory

I have an RAX20 router and I have a connected device which information is bugged in the router settings. I know the router must have some kind of memory for connected devices because it allows renaming them and that custom name will stick with the device once it's disconnected and reconnected. Basically, I want to make my router unaware that the particular device was ever connected to it and remove every trace of it.

To briefly explain the reason why I want to do this, the router defaults to an ancient hostname when trying to make a DHCP reservation and it will not allow me to enter the proper current one because it has too many characters (that also is a weird thing because fetching the hostname from the device which  was 18 characters long it would accept it but I cant enter anything manually with more than 15 characters). Sadly, because this device is managed by an IOT platform, I can't make the hostname shorter and applying a custom one in the router DHCP reservation that doesn't follow the same naming scheme as all my other similar devices would be a pain to work with.