The MR1100 DHCP offer's subnet mask (255.255.255.255) was causing the ER605 to decline the DHCP offer. TP-Link Tech Support sent a beta version of firmware which fixed the problem. If anyone else has...

@John1234 Good suggestion to contact the ISP, but I don't have the 7 or 8 hours that it would take to get through AT&T's customer/technical support to someone that might understand the problem, only...

@John1234 Thanks for the response. 1. No, I don't have another device using 10.247.116.156. The AT&T router is in "IP Passthrough" mode and seems to always assign an address to my router like...

Wireshark was used to show that the ER605 declines DHCP offers from a Netgear MR1100 LTE Router configured as an IP Passthrough device. My previous router (a TP-Link Archer C59 running OpenWrt)...