@JonSkeet , DHCP is being handled by the AX90 or by other device in the network?

  @Cattalins The AX90 is the only DHCP server on the network.

  @JonSkeet 

Are you running firmware 1.1.8?

The missing devices have static IP's?

Are they wireless or wired?

Are they connected direct to the router, or behind a switch, access point, or other network device?

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I saw similar issue when DHCP is OFF (instead of AUTO) and/or when the device had a static IP (instead of DHCP reservation)

  @Cattalins Can't check on firmware right now (on holiday). I believe all the devices involved are using DHCP though. Almost everything is behind another switch or two (lots of devices spread out around the house). Some alha e reservations, others don't.

  @JonSkeet 

I did not understand the last phrase (alha e?)

On AX90 you see all devices in the DHCP list, but they are missing in the network map, is that correct?

If the missing devices are being served by DHCP, you can try to reduce the DHCP lease time, so the list can be updated more often. 

(I do use 2 minutes + address reservation, and my network map seems quite right)

Note that if you reboot the router, devices that have fixed IP or address reservation, can be missing

I don't know a workaround for this, except to reboot the client too

In case they have static IP, ceck if network mask (subnet) is the same as the rest of the network (and they do belong to the same network as the AX90).

Perhaps you could have a loop in your network?

For example, you can have a loop when you connect your AX90 to both switches, and connect both switches together.

Or instead, if you connect the switches using two different ports at the same time.

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I did observe the missing network map when AX90 was in Router mode and DHCP OFF

I have three in AP Mode and DHCP AUTO

They are connected by a switch

DHCP is served by other device

  @Cattalins That was a typo - on mobile, didn't spot the mistake. Some have reservations, others don't.

Can't check anything else right now - on holiday as noted before, hundreds of miles from the router. Will look again when I'm back home.

  @Cattalins Okay, I'm back home now, so can check more things:

Firmware is now 1.1.8 Build 20220321 rel.13967(4555), so that's fine.

The DHCP lease time is 120 minutes, and the router has been up for far, far longer than that (some network map entries show a duration of 47 days, and the internet connection last changed status 17 days ago), so that really shouldn't be the issue.

Currently the network map is showing 23 clients, whereas the DHCP server page is showing 66 clients - that's a pretty large difference. There are no other issues in the network (and note that the AX90 is clearly *aware* of the missing devices as they're showing up on the DHCP server page).

This is only an annoyance rather than a major problem, so I think I'll just let it lie at this point.