What I'd like to do is to assign different DHCP to one of the Chateau 5G ports (i.e. ether2) to be isolated from other ports but at the same time to have WAN access via lte1 interface.
This is what I've done but connected receive their IP addresses assigned but there is no WAN access:

1) Created a new bridge: bridge2
This part I'm confused about as from what I've read seems to me like Chateau has some limitation with lte1 interface bridging but I might be heavily wrong, so this might be the very source of my problem.

2) Assigned IP -> Addresses -> Created a new address
Address: 192.168.100.0/24
Subnet: 192.168.100.0
Interface: bridge2

3) Created pool IP -> Pool:
Name: dhcp2
Addresses: 192.168.100.100-192.168.100.199

4) Created a DHCP server:
Name: dhcp2
Interface: bridge2
Address pool: dhcp2

5) Changed Bridge -> Ports:
Interface: ether2
Bridge: bridge2

Am I doing something wrong or this isn't possible with lte1 as WAN access interface?
I guess backup option would be to use the same DHCP as on other ports but to implement firewall rules to isolate this particular port.

Thanks.

On your step #2, set the IP address to something different than network address, so like 192.168.100.1/24.
What you want to do is doable with no issue at all and once the proper address is assigned ROS will create proper routing info the for the new bridge2 interface.