Go to your Gateway at
http://gateway.2wire.net (that is the local unit, and it is not going out on the internet for that). Username is admin, and the default password is written on your 3801HV.
I suggest going to Settings>LAN> IP Address Allocation
For Device: OBi202, I would select Address Assignment: Private Fixed for one of the addresses. I would use the one that ends in .202 to make it easy to remember. Firewall can be left at its default Enabled and WAN IP Mapping can be left at the default. Then click Done, or whatever the button at the bottom is. Expect to be asked for the password again. The point is that the OBi will always get the same address, so if the router reboots, the OBi will not be out of sync with the router. Then power-cycle the OBi202.
Things should work. If not, we can try other things.