Our ADSL is dropping constantly so Vodafone wanted me to replace my trusty TP-Link broadcom chipset modem with their Ultra Hub.  I've installed it and it's connecting, but I'm having a nightmare of a time connecting to the administrative interface from the internal lan.  

I have a Dual WAN setup and use a Meraki MX64 to manage all of our systems and traffic and the ADSL is used as my failover connection when my primary WAN (which is via a WISP) goes down.   As such, the Meraki gets a DHCP address issued from the ADSL modem and then double-NATs the traffic.   Definitely not ideal, but since NZ uses PPPOA for ADSL and there is no way to put this modem into bridge mode, a double-NAT scenario is my only option.  This worked perfect with the  TP-Link modem and it seems to work OK with this Ultrahub *except* for accessing the admin interface via the LAN.

If I wait about 5-10 minutes, the admin interface will load via a LAN connection, but then when I enter my user/pass it says invalid user/pass (even though I'm copy pasting) and then I get locked out and only get the 503 error when trying to load the  admin interface again???

If, at the same time, I connect to the ultrahub via it's wifi network and login using my phone or a tablet, I can get in no problem.

What is going on here?  I never had this issue with the  TP-Link sitting in the exact same configuration?  The only thing that I have changed on the Ultra Hub is I moved it's internal LAN IP to 192.168.254.1  (I didn't want it on 192.168.1.0/24 or 192.168.0.0/24 as to prevent any double-nat conflicts with the intermediate hardware).