Thank you for your query.
Indeed, your devices behavior is odd. Is there a chance you could download the troubleshoot file and forward it to me via private message please?
In order to download the troubleshoot file please navigate to System -> Administration -> Troubleshoot -> Download Troubleshoot file and download the file. Please make sure that the troubleshoot has been downloaded after the issue occurred and before any reboot/restart.
Regarding your firewall, you could simply test it out, disable it manually and check whether the issue persists: navigate to System -> CLI -> login root, password: router password. type in: /etc/init.d/firewall stop
This will disable the firewall completely, you can check whenever the packets are still being dropped afte 20 s. If not, then the problem is with firewall and possibly a traffic rule needs to be added.(But it is not very likely, because firewall wouldn't allow the traffic at all, it wouldn't stop after 20 s)
Please, keep in mind that disabling your firewall is not recommended and do it only at your own discretion. Don't forget to start the firewall again with the command: /etc/init.d/firewall start after you finish testing.