Connection problems and "WLAN access rejected: incorrect security" in logs

Hi - I'm having a lot of problems with an RAX20 (which isn't an option in the "Model" field, for some reason, so I chose the closest match. Firmware version is 1.0.2.64_2.0.42.) in Access Point mode. I'm seeing numerous log entries showing the same "WLAN access rejected: incorrect security" message that others have reported, all relating to MAC addresses which are recognised devices and which have the correct password. Here's a sample (with addresses redacted):

[WLAN access rejected: incorrect security] from MAC 00… , Tuesday, Jan 12,2021 16:08:24
[WLAN access rejected: incorrect security] from MAC 42… , Tuesday, Jan 12,2021 13:08:28
[WLAN access rejected: incorrect security] from MAC 00… , Tuesday, Jan 12,2021 09:39:17
[Time synchronized with NTP server] Tuesday, Jan 12,2021 08:07:40
[Internet connected] IP address: 192.168.0.85, Tuesday, Jan 12,2021 08:07:40
[WLAN access rejected: incorrect security] from MAC ec… , Monday, Jan 11,2021 23:52:53
[Time synchronized with NTP server] Monday, Jan 11,2021 20:07:39
[Internet connected] IP address: 192.168.0.85, Monday, Jan 11,2021 20:07:39
[WLAN access rejected: incorrect security] from MAC 42… , Monday, Jan 11,2021 17:56:17
[WLAN access rejected: incorrect security] from MAC 42… , Monday, Jan 11,2021 17:55:33
[WLAN access rejected: incorrect security] from MAC 00… , Monday, Jan 11,2021 16:34:13
[WLAN access rejected: incorrect security] from MAC 00… , Monday, Jan 11,2021 15:27:36
[WLAN access rejected: incorrect security] from MAC 42… , Monday, Jan 11,2021 13:05:21
[WLAN access rejected: incorrect security] from MAC 42… , Monday, Jan 11,2021 11:30:04
[WLAN access rejected: incorrect security] from MAC 00… , Monday, Jan 11,2021 10:22:20

Another symptom that I'm seeing regularly is my Mac being able to connect to WiFi, by not getting through the RAX20 to the cable router (a Virgin Media-badged Arris Touchstone) which is providing the DHCP service and so having a self-assigned IP. Sometimes this resolves itself after a while, but rebooting the RAX20 always seems to solve it. However, many devices are having connection problems at times, which is very frustrating.

The Home section of the Basic tab at routerlogin.net shows security as "Enabled", but I've turned off the only available option for security under the Advanced tab, Access Control, to see if it would improve matters (unsuccessfully). What other security features are running? It seems that they're preventing my devices from connecting through the RAX20 to the cable router. Can I turn them off, and if so how? If not I think I'll have to return the RAX20 as it's letting me down far too often.

Thanks.