You probably need to turn off power saving for this connection on your N900.

Some routers just don't work well with the N900's power saving feature, and this leads to a confusing situation where outgoing connections from the N900 work fine, but incoming connection requests from outside (such as SSH) work poorly or not at all.

It is a bit hard to find, but if you step through to the last page of the settings for this WiFi connection and press the advanced button and switch to the "other" tab, there is a setting for power saving. The middle setting isn't terribly useful so I suggest setting it off for this connection and leaving full power saving on for any other hotspots that don't cause problems.

Turning off power saving will significantly affect your battery life so you will probably want manage your connections manually or leave the N900 on a charger most of the time when it is in range of this router.