WNDR34700v4 - LAN speed decreases from 200+MB/s to less than 10 mb/s over days

IMO, and others also.. When a router's firmware is updated, it is always best practice to reset the router to factory default. Even more important if it has never been reset. Artifacts from previous version's of firmware can cause unpredictable problems, if not reset. It also allows for the drivers to be correctly loaded, as a opposed to an update and reboot.. I have been doing the "reset to factory default" since I first started with these routers, and have never had any strange issues, unless an issue is directly related to a firmware version, which can be quickly identified. Takes out the guess work..

EDIT: the config without AP mode by diabling DHCP could be attributed to an internal performance issue. I would start with the reset to factory first, and go from tthere..