You can find the VDSL Settings at Interfaces → Assign → VDSLs.
VDSL stands for Very High Speed Digital Subscriber Line and provides data transmission over the telephone network. The following options can be changed:
Management IP of the modem is used internally to read out connection data. You can change the local ip to reach it here in case it collides with your network. It must be an IP in the range of 10.0.0.0/8.
Multiplex can be either LLC (Logical Link Control) or VCMUX (Virtual Circuit Multiplexing). Default is LLC.
VCI is the Virtual Channel Identifier and 32 by default.
VPI is the Virtual Path Identifier and 1 by default.