Slow file transfer over wifi, but fast over internet

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Andy T11

Mar 9, 2011
I have a Tmobile branded Asus AC1900 router. Samsung Galaxy S8 phone with latest software updates The router is configured in Wireless access point mode. The router is from Verizon fios. I run speedtest and get 60-70 Mbps. Also, file download from the internet is fast. However, if I try to copy files from my PC/NAS using the WiFi network, to the phone I am only getting 1 Mbyte/sec at most.
The Wifi is using B/G/N mode with auto configured channel. I also tried using my original Verizon router's wifi set to G mode only and also had the same issue with slow speed.
How can I fix it?
I am able to copy files at full speed to other devices (tried Win10 tablet).