Why am I getting only 100Mbps out of my 500Mbps subscription?

You will need to replace your router with one that is Gigabit Ethernet capable.

Your Archer C50 AC1200 router spec page does not list it as having Gigabit, you only have 100Mbps ports:

The fact that you only have a 100Mbps WAN port means that it it the limiting factor to your internet speed. If you had a 1Gbps WAN port and 100Mbps LAN ports then at least you might be able to get an aggregate boost to speed (multiple 100Mbps devices/Wifi at the same time), but as it is your router is limiting your internet at its WAN port.

You might have better speed between Wifi devices though: