/quote "When the Obi202 is plugged in the LAN of the ER, I get the problems stated above but when I connect it to the upstream ATT modem/router LAN (ER's WAN) it works perfectly. So I am sure the problem is with the ER's config."/quote
You should probably try to put the "ATT modem/router" into Bridge mode. Having both the ER and ATT doing firewall and NAT is not good for anything and quite possibly your issue.
OBI 200 + ERL here with no issues although I did disable SIP_ALG.