Possible it's incompatible with the modem. Sorry that the 655 doesn't work for you. If the DIr-624 works well for you then ya, go for it. Just know that you'll only have the G band and no N modes for wireless, if this is ok for you then go for it. Should be good for you.
We could like to know if the 655 works at your sisters place and what modem she has on her system. Do you and her have the same ISP service? I wonder if maybe changing the ISP modem out thats the same as your sisters would be good too. If you want to keep the 655.
Please let us know what your sister has for a modem and if she has the same ISP.