Update
I have disabled the SIM switching on weak signal feature. Now watching the syslog in realtime I think the key issues are
Mon Oct 17 11:17:56 2022 daemon.info unhandler[2806]: [init_system:48] error: Cant init network subsys
Mon Oct 17 11:17:56 2022 daemon.info unhandler[2806]: [main:285] error: Can't get interface from network config
and
Mon Oct 17 11:18:18 2022 daemon.info sim_switch[15993]: [rules_check_data_limit:373] error: Failed to look up for `quota_limit object!
Mon Oct 17 11:18:18 2022 daemon.info sim_switch[15993]: [rules_check_sms_limit_reset:459] error: Failed to invoke `check` method!
Mon Oct 17 11:18:19 2022 daemon.info sim_switch[15993]: [rules_check_data_limit:373] error: Failed to look up for `quota_limit object!
Mon Oct 17 11:18:19 2022 daemon.info sim_switch[15993]: [rules_check_sms_limit_reset:459] error: Failed to invoke `check` method!
Mon Oct 17 11:18:29 2022 daemon.info sim_switch[15993]: [rules_check_data_limit:373] error: Failed to look up for `quota_limit object!
Mon Oct 17 11:18:29 2022 daemon.info sim_switch[15993]: [rules_check_sms_limit_reset:459] error: Failed to invoke `check` method!
Mon Oct 17 11:18:29 2022 daemon.info sim_switch[15993]: [rules_check_data_limit:373] error: Failed to look up for `quota_limit object!
Mon Oct 17 11:18:29 2022 daemon.info sim_switch[15993]: [rules_check_sms_limit_reset:459] error: Failed to invoke `check` method!
Mon Oct 17 11:18:39 2022 daemon.info sim_switch[15993]: [rules_check_data_limit:373] error: Failed to look up for `quota_limit object!
Mon Oct 17 11:18:39 2022 daemon.info sim_switch[15993]: [rules_check_sms_limit_reset:459] error: Failed to invoke `check` method!
Mon Oct 17 11:18:39 2022 daemon.info sim_switch[15993]: [1-1.4] SIM switch in progress. Triggered by data_fail rule
The thing I don't understand is why suddenly the data rule fails - both SIMs have normal data available to them as they did before.
But clearly there is a problem with "Can't init network subsys"
Also in reference to the quote limit error:
~# cat /etc/config/quota_limit
config interface 'mob1s1a1'
config interface 'mob1s2a1'
option wrn_sent '0'
option sim '2'
option modem '1-1.4'
option ifname 'mob1s2a1'
option previous_period '3'
option previous_reset_day_hour '1'
option due_reset_time '1667214000'
option enabled '0'