This is pretty strange. I have the 4582 modem in Bridge mode with an 86U behind it in full router mode. If I flip the modem to Gateway mode or reverse the 86U picks up its new address without any issues.
Consider rebooting the 86U after you have flipped the modem into Bridge mode, just to see if that makes a difference.
Also, are you using a cloned MAC address or one that you created in the 86U?
On the Advanced Settings .... Wan tab, check the following:
WAN MTU: 1500 That is what I use with the 4582 modem
DHCP query frequency: Aggressive Mode or Normal mode set ? I use Aggressive Mode
Consider flipping the mode to the opposite mode of what is currently in use.
If nothing works, then the only remaining conclusion is that your model of the XB6 is not compatible with the 86U, and possibly other Asus routers or with Asus routers running Merlin's Asuswrt. That has happened previously with the 4582 modems and D-Link routers, so, there is always the possibility that you're running into a similar situation. If thats the case, look at the bottom of the modem. It will either indicate an Arris TG3482ER, or a Technicolor CGM-4140COM. Call tech support and ask for a tech visit to swap the modem to the other model, and ensure that the visit won't cost anything. It shouldn't. My choice would be the Technicolor modem, so, hopefully you have the Arris modem and will end up swapping to the Technicolor modem.
My previous cable modem was the same as yours, a Hitron CODA 4582. I had no problems what so ever with it! But Rogers replaced it with the new ignite system.
All I was trying to do was lower my monthly Rogers bill. Because they're phasing out the old digital system, I had to go with the new ignite sytem.
I want it back... 😞
I did reboot the 86U after flipping to Bridge mode.
No cloned MAC
WAN MTU: 1500
DHCP query frequency: Normal
I'll try flipping the above setting to aggressive.
Here's what I have set for my 86U which is a little different than the post @-G- referenced:
DHCP query frequency Aggressive Mode
Extend the TTL Value: No
Spoof LAN TTL Value: No
Food for thought, if and when you have time, load the stock Asuswrt to see if that makes any difference. If not, then there is an incompatibility between your model of the XB6 and Asuswrt. If stock firmware works, then there is some change in Merlin's Asuswrt that is creating a problem.
Have you taken a look at the Syslog to see if it contains any hints that might point to the problem?
Technicolor modem, hmmm. I had high hopes for that modem.
The Arris and Technicolor XB6's each have their pros and cons and their own unique (and sometimes frustrating) quirks. However, the XB6 itself (in Bridge Mode) really should not be playing any part in this not working.