Hello,
Note: Kindly upgrade to the latest Firmware, you can find it out here https://wiki.teltonika-networks.com/view/Main_Page
Please keep in mind that I tested it myself and it works perfectly.
Kindly note the following points:
Go to IPv4 netmask > Click on Custom and enter your Subnet mask, for example, 255.255.255.240
If the DHCP server is enabled, the IP address range should be between 14 networks, as if we were using 255.255.255.240, which means /28 = 14 networks, for example. 192.168.1.1 is the starting IP, while 192.168.1.14 is the ending IP.
Then click Save & Apply and wait a few seconds for the changes to take effect.
Please let me know if this issue has been resolved.
Best Regards,