@anthonie, thanks for the input. I'll just try the video instructions in SHARP66's link before giving up.

EDIT: nothing worked... i just installed DD-WRT. Will try tinkering with the modem at home and report back if it fixes things.

UPDATE: DD-WRT didn't help. I flashed back to the original/stock firmware, updated it and tried to set everything up again according to the instructions posted in the OP and it worked!
But the N900 cannot connect. All other gadgets I tested were able to working but it doesn't matter since my in-laws do not have an N900 in their house.