I currently have the Nighthawk M1 setup as:

Gateway IP address: 192.168.1.1
IP subnet mask : 255.255.255.0
DHCP starting address: 192.168.1.30
DHCP ending address: 192.168.1.99
Use mobile router as DHCP server: Enabled.

The Asus DSL AC68U is connected to the M1 via LAN cable. Its settings are:

DHCP server: Disabled
LAN IP address: 192.168.1.25
Subnet mask: 255.255.255.0
IP pool starting address: 192.168.1.1
IP pool ending address: 192.168.1.254

With this configuration the PC works!

The Gigaset settings are:
IP address type: Static
IP address: 192.168.001.020
Subnet mask: 255.255.255.000
Default gateway: 192.168.001.001
Preferred DNS server: 192.168.001.001

With this configuration the Gigaset does not work!

I am way out of my sphere of experience here so I am asking for as much help as I can get.