Two things that may help further diagnose your problem.

Try temporarily putting your N900's IP in a DMZ on you home network, this will rule out firewall/port issues with the router on your internal IP.

Try switching off power saving on your AP in the N900's settings.
Settings > Internet Connections > Connections > (Your AP's Name) > Edit > Advanced > Other > Power Saving

With my router I get trouble with SSH and e-mails if the power saving is on max. This also only happens on my home network.

It may not work but it should rule out the router as the issue.