You aren’t trying to send 4K HDR/LLDV signals @120Hz. If you did try, it wouldn’t work. The 18Gbps spec cannot support more than 4K @60Hz and color sub sampling of 10 or 12 bit color is needed then as well. Those wanting to send 4K @120Hz HDR RGB/4:4:4 10 bit or LLDV 4:2:2 8 bit video signals as well as uncompressed multichannel audio signals will need much more bandwidth and the HDMI 2.1 ports in use must be set to 8K Enhanced.
Now, if every device in the chain supports DSC, then the signal can be compressed enough to use 4K @120Hz if the HDMI 2.1 ports support 24Gbps minimum. Information regarding every device in the chain here would be helpful
@randomuser.