Hi Peter,
Assuming that the 2nd WiFi AP (TL-WDR3600) is acting as a router and giving out it's own IP addresses which will place the laptop on a different subnet (eg 192.168.x.x vs main 10.0.0.x). This can have a negative impact on file sharing, so it should be avoided.
The best suggestion would be to disable the DHCP server on the WDR3600 and connect it to the TG797 modem from the LAN port (not WAN). This should make it act as a tranparent bridge rather than a router and the laptop should get a 10.0.0.x IP from the TG797 which should allow the file sharing.
I hope this helps.
- Shelly