Network transfer speeds slow across Access Point

Hey folks,

Trying to solve a Network Transfer issue. Setup is as follows:

1 optimum modem in Bridge Mode, 

Router is an R7000AC 

          Which has wired connections to:

          -15' cat6 cable connecting to Computer A (10/100/1000 LAN)

          -50' cat6 cable connecting to an R6700AC in Access Point mode

                    -Wirelessly broadcasting to Computer B (802.11AC, 5GHz, 585/650Mbps)

Transferring large files from Computer B to Computer A max out at about 11MBps, which is more like 802.11b speeds. What settings can I check to try and get the most out my network? I will be regularly transferring large files. Computer B is located in a way where hardwiring is not possible. It is a very recent machine, with built-in WiFi 6 AX200 1600MHz capabilities (though that means nothing connected to the R6700). It should be capable of much faster transfer speeds, no?

Cheers!