It seems this solution may work, depending on your provider I suppose :
- Log in to your router via CLI (more details here: Command Line Interfaces)
- Run the command
vi /etc/config/network
- Locate
config interface 'mob1s1a1' and press “i” to enter edit mode
- Add the line
option dhcp '0' under the mobile configuration section
- Press “Esc”, then type
:wq and hit Enter to save and close the file