|3. Go back in again. Set IP to 192.168.3.1 (the N300 is at 192.168.2.1). I turn off DHCP. Reboot. Can't reconnect. Reset it.|
This is your problem 192.168.3.1 and 192.168.2.1 are completely separate networks (if you're using the following Subnet Mask: 255.255.255.0) and as such, can't "speak" to each other.
Try this instead, change the N150's LAN IP to: 192.168.2.2 and see if that doesn't get it working properly.
I selected 2.2 because most SOHO router's have IP's between .100 and .200 reserved for their DHCP scope.
.2 would be outside that scope (therefore ruling out a potential duplicate IP) yet still within the same subnet (network) as the N300 router (192.168.2.1)
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.