Are there sync logs available from the mobile application?
Do you mean logs of the sync attempts? I don't think these are available though I can't think what you'd need them for.
Re kafka not working, have you confirmed that Zookeeper is running? I assume you've checked in the Kafka logs for any useful output (/opt/kafka/logs/controller.log).
It might also be useful to check the broker details in Zookeeper:
$ cd /opt/kafka/bin
$ ./zookeeper-shell.sh localhost:2181
> ls /brokers/ids
[1, 2]
> get /brokers/ids/1
{"endpoints":["PLAINTEXT://10.xxx.xx.170:9092"],"host":"10.xxx.xx.170"}
> get /brokers/ids/2
{"endpoints":["PLAINTEXT://10.xxx.xx.171:9092"],"host":"10.xxx.xx.171"}
Ref: https://www.baeldung.com/ops/kafka-list-active-brokers-in-cluster