XGS firewall log

Hello,

Thank you for contacting Sophos Community!

The firewall has allotted limited space for the log viewer and hence we shall not have older logs in it. It is better that you review the Reports and may have better details in it.

Can I check the logs through the/var/tslog directory? Can you tell me if there is this log file

The Packet Capture Log is not available as a Logfile. 
Could you give us some more insight, why you need this? 

Bare in mind, Central Firewall Reporting supports a more detailled view on this one.

The Logviewer is more a tool for "live debugging" and not a review of connection of the past 30 days. 

I use DNAT and the firewall's rule has enabled logging, but sometimes I find that the firewall does not have relevant access logs, so now I need to view the logs to analyze

Try to take a look into Central Firewall Reporting.

Because it sounds to me, you are more interesting in Reporting than logging. Especially as logging only gives you entries, while this sounds like you want to have an overview to analyze.