Large data transfer hoses 5ghz radio

I recently got a new laptop and I'm trying to transfer data between my old one and new one.  I have my router setup with separate SSIDs for each radio.  If I have both laptops on a 5ghz radio I can communicate fine, to the internet and between devices, but once I start a file copy whichever radio my new laptop is on stops working after a minute or so.  All devices that were using the given radio get disconnected.  The SSID is still being broadcast, but Windows 10 reports 'can't connect to network' when trying to connect, tried forgetting, etc. same happens.  Reboot router and all is well again until I try a file transfer.  If I'm working with a 2.4ghz radio on either device it goes on fine, but of course is slow as hell.  I've rolled back firmware, factory reset, no change in behavior. 

Laptop has latest drivers/bios.  It's a ROG Zephyrus S17, with a MediaTek MT7921 card.  Tried setting it to .11ac.  Again, not change.  Really annoying.