According the specs on the Samsung au site https://www.samsung.com/au/smartphones/galaxy-a20-a205/ the A20 is "802.11 b/g/n 2.4GHz"; i.e. no 5GHz interface. You have to go upwards one model in the range - A30 - to get WiFi ac.
A short update for those that use a FAST5366TN-A with a Windows 10 client with Intel 9560 chipset.
Change the Intel 9560 driver setting from "auto" to 20Mhz. For some reason between the two they get confused and start dropping packets...
Unclear as to which end has the issue but the change makes the connection stable, although slow.