@KOKOSI 

500mbps sounds perfectly correct for an AC1200 WiFi connection, the AC (WiFI5) hardware is capable of 1200 with 3x3 connection, but most devices are just 2x2 and therefore the max speed is 866.  In real world terms this is around 500mbps

Are you testing this using a laptop or phone?   Are you connected wirelessly?

  @Philbert 

Everything is on LAN cable connection
from Main Rounter (1st Floor) >LAN> To My Rounter (2nd Floor) >LAN> to My PC.

To be clearly.
When I set to ROUNTER MODE I only got fully speed from ISP package. But when Change to ACCESS POINT MODE > Half Speed Reduce (I don't use WIFI because I need stable connection) Thank you

  @KOKOSI 

Hi,

Since you are in Access Point mode and in case you currently have the cable coming from your main router plugged into the blue "Internet" port of the Archer C1200, then you could try if you maybe get a better throughput if you plug that cable into one of the C1200's orange Ethernet ports. 

LAN-to-LAN port (orange) connections should be directly routed through the internal Ethernet switch chip, while the "Internet" port is normally wired to the CPU SoC and so the data has to pass through that CPU chip as well.

  

This is How I connect LAN

 

  @KOKOSI 

Like I mentioned earlier, since you are operating the C1200 in AP mode you can do the following test.

1) Remove the cable "Input From Main Router" from the blue port.
2) Now plug that same cable into any of the free orange ports. (# 2, 3 or 4)
3) Then run your "Internet speed test" again.

If you still get only 500 Mbps, then plug the cable back into the blue port and wait if perhaps someone else can come up with other suggestions what you could try. 

In case the Internet speed does increase if the "Input From Main Router" cable is plugged in an orange port, then maybe just leave it like that and be happy.