I was able to carry out a search and according to the manual, this is exactly how it should be done. According to the manual:
Click on Router. Then enter the following information before clicking on Apply:

IP address: Enter the new IP address of your router.
Subnet mask: Enter the subnet mask IP address
DHCP server: Activate the DHCP server
DHCP IP pool: Enter the start and end IP address for your DHCP.
DHCP time: Enter the maximum lease time in hours.

Unfortunately, it is not written why DHCP should not be adjustable. In this case, I would recommend contacting the manufacturer directly. I am also releasing the question again, as someone may have this router and would like to share their experience with us.