The key is to research the router, or modem/router, or WiFi unit before you buy.
I was lucky in that the kit I bought for here happened to be OK, and worked well together first time off.
Both my daughters had terrible trouble, and had to junk the first lot, do the research, and buy again. Number 2 still can't do MSN videoconferences because the NAT firewall in her modem/router won't allow it.
The key principles we came up with were:
1. Don't touch Belkin
2. Check online to find if the items you decide to buy are compatible with each other.