Lenovo Yoga 9i 2-in-1 Gen 9 external monitor is not detected or uses the wrong resolution

The Yoga 9i 2-in-1 Gen 9 has two Thunderbolt 4 USB-C ports that support DisplayPort 1.4, and an external monitor problem is usually the cable, the port or the display settings. For a monitor that shows nothing, connect it to the other Thunderbolt port and use a cable rated for video: a USB-C cable that carries only charging and data will not output a picture. Press Windows key plus P and choose Duplicate or Extend. For a flicker or the wrong resolution, set the refresh rate under Settings, System, Display, Advanced display, and try a lower setting, since a high refresh rate over a long cable can drop out. A dock or hub may limit the bandwidth, so test with a direct cable.

Update the graphics driver and the BIOS. On HDMI through an adapter, make sure the adapter supports your resolution. If a monitor works for a few minutes and then goes blank, heat or a marginal cable is the cause, and replacing the cable with a short certified one usually fixes it. Docks that supply 100 W of power and several displays need a Thunderbolt-certified cable, so use the one that came with the dock. On a Thunderbolt dock the laptop may also be driving the dock's USB devices and Ethernet, so a dock firmware update from the dock's maker can fix a monitor that blinks when the laptop wakes. Wake the laptop from sleep with the monitor already on to see whether the dropout depends on the order.