That post configures the ERL in bridging mode. The ERL simply isn't suitable for that. Don't buy an ERL if you need to use it in a configuration that it can't offload and expect more than 100Mbps performance. It's got a minimal CPU, so yes, performance will suffer if it can't offload.
You don't need to use bridging mode to bypass the AT&T RG. That post probably predates the EAP proxy solution.
https://github.com/jaysoffian/eap_proxy