Ok that’s even more complicated than I thought. I’m not sure you are going to be able to set a port forward rule to 10.0.0.x on the at&t router as it’s local range is 192.168.1.xxx.
This might work:
- on the at&t router set a port forward rule: ip address 192.168.1.xxx (r7000 ip), external port 443, internal port 8237
- on the r7000 set a port forward rule ip address 10.0.0.x (HA ip), external port 8237, internal port 8123
Edit: though if it is truly pass through from the at&t router you may only need the following on the r7000:
ip address 10.0.0.x (HA ip), external port 443, internal port 8123
This assumes you are connecting the at&t router to the r7000 WAN port.
Someone with more networking knowledge than me may want to correct this.
It would be a lot simpler if we could just change the DHCP range of the at&t router. What model is it? Usually it’s just under Settings / LAN / DHCP / Configure Manually.