Asus RT-AC66U issues

I am having issues connecting to my ssh server back home and I think its the Asus router which is causing this. Most of the time everything works normally. but there are times when the ssh connection fails, this started happening recently. I was able to connect remotely till yesterday evening but today I can't.

How can I troubleshoot and confirm that its the router which is causing the issue?

[Speedport modem]----->[AC66U]---->[HP Switch]---->[SSH PC]

Thanks