The fault was with DHCP server on the Hub. It wasn't issuing IP Addresses. When a PC was requesting an IP address, the Hub would just cut the device off when the lease time had expired, although the PC still had a valid IP address). The PC would then try to contact the DHCP but access was denied. Next the PC tried to contact 192.168.1.249 requesting an IP Address. There is no DHCP server on 192.168.1.249 (This is another device on my network - Yamaha YWA-10 Wireless Network Adapter with no active DHCP server on it). The only way to get the PC back on the network was to disable the network card and then enable it, and then the Hub would allow access to the network.
The problem was evident on all devices on the network (4x Windows 10 PCs, 5x Phones/Tablets with varied versions on Android), so it was not a problem with these. The Yamaha YWA-10 Wireless Network Adapter was temporarily removed from the network, but the problem was still there, so it is not related to that device.
Searching the Windows 10 Event Log, the errors about DHCP were only present when the above problem started happening. All the PCs, Android devices and the Yamaha Wireless Network Adapter have been on the network for a considerable time with no problem.
4 days ago, I set the Hub to factory default for the second time. This time, the errors have now disappeared. There have been no errors with DHCP & there are no sudden disconnections from any of the devices, so hopefully this might have solved the problem. Incidentally, the problem with 5G wireless has also disappeared, although this might or might not be related to setting the Hub back to factory default (I will keep monitoring this).
Yesterday, I removed the 10dB attenuator. Below are the new readings for downstream. Not sure if new readings are now within spec? The attenuator was fitted many years ago when we had some ancient modem fitted. Not sure if it is still applicable now - when we got the Hub 3 a few years ago I left it attached to the cable and just screwed it into the new hub with the cable?
Downstream bonded channels
Channel Frequency (Hz) Power (dBmV) SNR (dB) Modulation Channel ID
1 299000000 -1.5 40 256 qam 21
2 139000000 0.2 40 256 qam 1
3 147000000 0.2 40 256 qam 2
4 155000000 0 40 256 qam 3
5 163000000 0 40 256 qam 4
6 171000000 -0.2 40 256 qam 5
7 179000000 -0.4 40 256 qam 6
8 187000000 -0.5 40 256 qam 7
9 195000000 -0.7 40 256 qam 8
10 203000000 -1 40 256 qam 9
11 211000000 -1.2 40 256 qam 10
12 219000000 -1.7 40 256 qam 11
13 227000000 -2 40 256 qam 12
14 235000000 -2.2 40 256 qam 13
15 243000000 -2.7 40 256 qam 14
16 251000000 -3.2 40 256 qam 15
17 259000000 -3.7 40 256 qam 16
18 267000000 -3.4 40 256 qam 17
19 275000000 -2.4 40 256 qam 18
20 283000000 -2 40 256 qam 19
21 291000000 -2 40 256 qam 20
22 307000000 -1.5 40 256 qam 22
23 315000000 -1.5 40 256 qam 23
24 323000000 -1.9 40 256 qam 24
Downstream bonded channels
Channel Locked Status RxMER (dB) Pre RS Errors Post RS Errors
1 Locked 40.9 613 0
2 Locked 40.3 160 0
3 Locked 40.3 319 0
4 Locked 40.9 419 0
5 Locked 40.9 464 0
6 Locked 40.9 418 0
7 Locked 40.9 388 0
8 Locked 40.9 389 0
9 Locked 40.9 365 0
10 Locked 40.3 422 0
11 Locked 40.3 458 0
12 Locked 40.3 534 0
13 Locked 40.3 567 0
14 Locked 40.3 575 0
15 Locked 40.3 590 0
16 Locked 40.3 601 0
17 Locked 40.9 599 0
18 Locked 40.9 601 1
19 Locked 40.9 599 0
20 Locked 40.3 596 8
21 Locked 40.9 604 0
22 Locked 40.3 620 0
23 Locked 40.9 628 0
24 Locked 40.9 628 1
