Thank you for your prompt reply.
I checked the certificate using the commands you gave,
mosquitto_sub --cafile ca.crt --cert test.crt --key test.key -q 1 -d -h domain.name.com -p 8885 -t "/my/publish/topic"
The command above returned
Client mosqsub|766-king sending CONNECT
Client mosqsub|766-king received CONNACK
Client mosqsub|766-king sending SUBSCRIBE (Mid: 1, Topic: /my/publish/topic, QoS: 1)
Client mosqsub|766-king received SUBACK
Subscribed (mid: 1): 1
Client mosqsub|766-king sending PINGREQ
Client mosqsub|766-king received PINGRESP
Then I did
mosquitto_pub in another terminal
mosquitto_pub --cafile ca.crt --cert test.crt --key test.key -q 1 -d -h domain.name.com -p 8885 -t "/my/subscribe/topic" -m "hello there"
Client mosqpub|1567-king sending CONNECT
Client mosqpub|1567-king received CONNACK
Client mosqpub|1567-king sending PUBLISH (d0, q1, r0, m1, '/my/subscribe/topic', ... (11 bytes))
Client mosqpub|1567-king received PUBACK (Mid: 1)
Client mosqpub|1567-king sending DISCONNECT
Does that indicate the MQTT connection is established correctly?
The LTE Link Monitor however still outputs
[00:18:48.585,418] [1;31m<err> mqtt_simple: mqtt_connect -45
[00:18:48.585,449] [0m<inf> mqtt_simple: Reconnecting in 60 seconds...
Does this means the certificate is still wrong?
Thank you for your help.