Why does the Secondary DHCP server in the modem affect the speed?

Question

Computer networks

There is fiber optic internet with a speed of ~100 Mbps.
There is a HUAWEI HG8245 modem.
In its settings, 2 dhtsp servers are enabled, it is not clear why. Because of the second, the speed drops somewhere to 10 Mbps.
When the second one is turned off, the speed again jumps up to 100.
And then there are two problems:
1) Everything would be fine, but when the modem is rebooted, this second one turns on again.
2) The speed of the Internet for devices connected via Wi-Fi in any case is not more than 20 Mbps.
How to solve these problems?
Screen below