Hello @shimi 

The Zyxel has 192.168.1.1 as it's LAN IP address, I read on the internet that the UDM has the same IP, try changing it to 192.168.0.1 for example.

Hi @Waqqas ,

Thank you for you reply.

The UDM is already using IP: 192.168.2.1 (called Gateway IP in the UDM controller. I don’t remember if it was set to this value automatically or I set it in the past), and the ZYXEL assigns the address 192.168.1.2 to it (using static DHCP).

So typing 192.168.1.2 in the address bar takes me to the UDM device, and typing 192.168.2.1 takes me to UniFi networks portal.

Now when I go into the networks section in the UDM I have two networks: LAN and WAN. The LAN IP is 192.168.2.1/24 and the WAN IP is 192.168.1.1/24.

That seems wrong to me, but I have no idea what the WAN should be (192.168.1.1/24 was recommended by the app)

Thanks again, Shimi

@shimi, did you get this figured out? I am thinking of buying a UDM soon but want to make sure it works with t-mobile 

Hi @dawn_chorus 

I tried both solutions:

• I set a VLAN at 192.168.10.1. You will need to move the wireless and all ports to use VLAN (but not the WAN!)
• I changed my regular LAN to 192.168.20.1 (using 192.168.2.1 wasn’t recommended in an answer elsewhere)

So far no problems, but I restarted everything yesterday, so it usually takes a few days to notice the problems.

Another proposed solution was to buy a Daytrek modem. I looked into it and it should cost about 100€, and you can put it into a bridge mode. 
Looks like a good backup plan.

if you find another solution that works, please post it here.

Cheers 

@shimi

Two routers behind each other is never a problem, the only thing is to remember is to have them in separate sub nets. LAN DHCP’s should not be in the same sub net.

The Zyxel should be called with 192.168.1.1 and the UDM should be called with 192.168.2.1 (downstream connected), If you call 192.168.1.2, you connect to the WAN of your UDM and you enter the UDM from the WAN side (hairpinning your UDM) 192.168.1.2 is the assigned WAN IP towards the UDM by the DHCP of the Zyxel in the sub net 192.168.1.xxx. The Zyxel has its DHCP working from 192.168.1.2 till 192.168.1.254 (if you did not change that).

Calling the router should always being done via the xxx.xxx.xxx.1 of the net used, mostly never by the xxx.xxx.xxx.2 or higher, because that is reserved for the DHCP range.

@Pieter_B 

Thanks for the details.

It looks like you describe exactly what I did:

Zyxel IP 192.168.1.1, Zyxel LAN1 connected to UDM WAN, and Zyxel assign 192.168.1.2 to the UDM. One note: the MAC address the Zyxel see is for eth4 on the UDM, but the MAC the UDM shows in the unifi app is of eth0 (checked with SSH to UDM and ifconfig). I don’t know if that matters or not.

The UDM router was on 192.168.2.1 (and all clients where on 192.168.2.xxx)

Typing in the address bar 192.168.1.1 takes me to the Zyxel page, and 192.168.2.1 to the UDM page.

So it looks like it is configures ok, but still the setup don’t work properly (many drops and random disconnections)

@shimi 

You mentioned that you changed the gateway to 192.168.20.1 instead of 192.169.2.1

Good to hear that everything is working smoothly till now with this change, let's hope it stays the same.

Let us know!