Netgear XR500 Unnamed devices poping up

Apologies for the repeat of an earlier post, but I'm still having the same problems with my XR500. 

I hadn't checked in on routerlogin.net dashboard for a few months, and when I checked the Device Manager, there were a lot of 'unnamed devices' that I didn't recognise.

I went around the house and made a list of wifi-enabled devices we have, and checked them off against the devices that were showing up on the routerlogin.net dashboard (I didn't check using the IP/MAC address but most of the devices in my house have names that were imported from their individual device settings - for example, my iphone was called MYIPHONE7). So I was able to confirm that none of these 'unnamed devices' on the dashboard corresponded to actual devices in the house. 

So I figured that they were probably guest devices that had logged onto the wifi network over the last number of months, so I deleted most of them. However, I ran into two problems:

1) There were 2-3 of these 'unnamed devices' actively logged onto one of the wifi networks so I got an 'Unable to delete device beccause it is online' error message when I tried to delete them (even though again - none of these corresponds to actual physical devices in the house)

2) When I checked back in a few hours later, there were several new 'unnamed devices' in the offline section of Device Manager. I switched the XR500 off and on again, but this did nothing. 

There are currently 10 unnamed devices, of which 7 have the same first six digits in their MAC addresses - the other three share a different first half. None of the unnamed devices share the first half of a MAC address with any of the actual physcal devices that I know should be there. 

Can anyone please advise on how to fix this problem?

Many thanks