Peplink Issue - TM

Customer have an issue regarding Peplink product Model Balance 20X with firmware 8.0.2 1045. Our service is Broadband ™ with 800Mbps Download and 200Mbps upload. The issue is the throughput drop around 200Mbps instead of 800Mbps for download and for the upload is normal. They did the Speed test a few times, but the result still persist as mention. The continuous ping is normal as well. The configuration for WAN Mode is PPPOE. The configuration at Peplink Balance 20X for WAN Mode is PPPOE. The have done a different method for equipment set up. They configure Router Gateway as a PPPOE and change the WAN Mode to DHCP at Peplink Balance 20X, the Download can reach up to 800Mbps, but the continuous ping is intermittent.For your information, the going to bypass the Router Gateway and replace to Peplink Balance 20X. Kindly, please advise whether can resolve the issue

Just thinking… When you change the Peplink to do the PPPoE… Did you change the device of your ISP to bridge mode or you keep it in router mode?
Here in my country… When we do PPPoE with Peplink. We change the ISP device do bridge mode and disable the WiFi of the ISP device, also. Doing this, we always getting better performance.