MR1100 from ATT- mobile broadband disconnect message when connected to home router

[goyerag]

Originally I had the MR1100 configured as a modem only (ATT) and connected via ethernet cable to a home router. Everything had been working fine. I had changed the following setting in the MR1100 from the factory setting to allow the MR1100 and home router to communicate (I did this b/c my home router has a much larger range than the MR1100).
1. Turned on IP passthrough
2. Turned off DHCP
3. Ethernet standby set to disabled

On Tuesday morning the internet abruptly stopped. When I checked on the modem it had an orange flashing light and was giving me a message that the mobile broadband was disconnected. After several hours of troubleshooting with ATT (along with a new SIM card) it still was not working and they were out of ideas. I decided to press the factory reset button as a last resort and the communication to ATT began working again. I was able to directly connect my devices to the MR1100 WIFI and the internet worked. I then attempted to modify the MR1100 configuration to allow the router to be reconnected (same settings as listed above). As soon as the settings were changed and MR1100 rebooted, I again received the orange light and mobile broadband disconnected message. I again disconnected the router, reverted back to the factory settings, and it immediately reconnected with ATT and has been working.

The only thing that changed (that I can see at least) is a firmware update on the MR1100. I believe the modem updated to firmware version 12.06.21.00. I cant confirm when this firmware was released or installed but it appears about the same time as my problem started.

Any suggestions on how to reestablish being able to utilize my home router to increase WIFI range?