Does the MS350 have a dedicated management port? From what I see of the specifications, it doesn't.... not like a MS410 has.
But anyhow, like I've said in my post about my MS320s, Meraki has got to fix their terminology in this regard. They use the terms Management, LAN IP, and Uplink interchangebly, and if you're not familiar with Meraki equipment you can get very confused.
It's also confusing that you can set Management VLAN in the Configure>Switches section, but then bypass that by specifying a different VLAN ID in the LAN IP, AKA, 'Uplink configuration', AKA, 'Uplink Internet Connection'.
The actual Uplink Interface is located in the Configure>Routing and DHCP section. That's where you specify the Uplink Interface, it's gateway, and VLAN. That CAN absolutely be in the same address range as your Management VLAN.
For instance, my MS320s are configured like this- Uplink Interface= subnet xxx.xxx.xxx.136/29, IP of xxx.xxx.xxx.138, a VLAN ID, and then the gateway/next hop is xxx.xxx.xxx.137. After that, I set my Management/Lan IP as xxx.xxx.xxx.139, same VLAN ID, and xxx.xxx.xxx.137 as the gateway. My situation is a little different, because I don't have a direct route to the internet, or even the final hop router just downstream of the internet gateway.
In your case, if you're connecting directly to the AT&T and you're capable of administering it, adding VLANs to it, etc... then you should have an easy task of creating a VLAN11 on your desired subnet. Even if you have a L3 in between, it would be the same. The first interface you create in the Meraki dashboard should be the VLAN1 information, because it will automatically creat the route for you. Then, create a second interface with your VLAN11 information as that is going to be your way to the cloud. Here's where it gets tricky.... you CAN use the same IP of the VLAN11 interface as your management/LAN IP/Uplink configuration, but it would probably be better to use a 3rd IP for it in case they change it down the road (as it seems to be unintended to work with just 2 IPs). Connect your uplink and you should go green and pull down all the dashboard configs.
Confused? You'll get used to it!