@Joseph-TP 

RE220 V2.0 1.0.2 Build 20201022 Rel. 66896

RE305 v3.0 1.1.5 Build 20210416 Rel. 61543

The fault is different than I thought,  both of the router's Onemesh menus (Basic -> Network Map -> "Mesh Devices" AND Basic -> "Onemesh") are usually only displaying one Onemesh device at a time.  Both extenders are fully operational and clients connected and talking through both.  When I say usually, tonight is the only time I have seen it display both extenders since the router firmware upgrade earlier this year.  Both extenders have 3-bar signal strength on the router and a solid throughput.

Both extenders now appear in the tether app at the same time even when router is still showing only one.

20 minutes after switching Onemesh on, the router still wasn't displaying both Onemesh devices even though both have been operational very soon after switching on again. Initially it showed the RE220 but it was disconnected and "failed" when I added it, then the RE305 showed as connected and the RE220 disappeared. EDIT: 30 minutes after and both now appear again.

The Tether Lab "Wi-fi Detective" can't see my RE220 even though my phone is working through that extender, it does see the RE305.  I guess this might be expected behavior with Onemesh.

There is nothing in the router logs of relevance (only has LED controller INFO messages), extenders don't appear to have their own logs, or at the least I haven't found them.

Just in case it is of any relevance, this is entirely a domestic TP-Link network, with one router, two Onemesh extenders, four Gigabit Powerline adapters and three Gigabit Ethernet switches driving worst case about twenty devices. About fourteen devices are near enough 24/7 on Ethernet and up to about 6 Wireless devices intermittently. It sounds a lot for a non-premium router but I have never seen CPU or Memory report have any concern, the network is very smooth and high speed.  Memory is always less than 50% and highest CPU core about 35% (normally less).  The router WAN is connected to a IPv4 device in modem mode.

  @Joseph-TP 

It happens either way around, just the RE220 displaying or just the RE305 displaying on the C6 router onemesh lists or app.  As I've now confirmed they are both working even when not getting displayed, I assume the fault is on the C6 router and it is being temperamental about listing them even when it is satisfactorily managing them operationally.

I will hard reset all the devices in the next week.

I have spotted something else that may vaguely be related to this problem, I can no longer access the modem through its local IP any longer, I suspect this may be because I switched off all ALG settings apart from IpSec.  Could one of the ALG settings be interfering in the onemesh operation?

  @Joseph-TP 

Thanks, originally it didn't display both extenders at the same time for at least weeks and yet now the second one appears within the hour. The problem changed from a permanent problem to a temporary (intermittent?) problem so the results of any tests would not be entirely conclusive. As said, Onemesh is fully operational it is only a display problem.

Unfortunately I updated the firmware roughly at the same time as switching off most of the ALG functions so I can't say which triggered the problem.

I'll try the ALG functions first and see if that improves things.  I have noticed something else in that the router no longer bridges through to the modem's control panel which could be an ALG needs switching back on,

I'll play and test when I can, I've had a busy week shuffling paperwork.