Turns out an Nvidia firmware update is the culprit. In short, make sure you have the correct cables plugged in between your Thunderbolt port and monitor, install a Dell this firmware update,  then uninstall and reinstall the Nvidia drivers. Works like a charm! Detailed explanation of my adventures in computing below... Just to be safe, I got a USB-C to HDMI adapter that supports Thunderbolt 3 and above (cheap adapter from amazon ). To use the HDMI cable, especially since my Thunderbolt port is signal only (does not deliver power on the G15 5511), I got one with power pass-through. I hooked up the hard-wired USB-C adapter cable to the USB-C Thunderbolt, then plugged another USB-C connected to a power source (USB-A to USB-C from an externally powered hub) into the adapter's USB-C power input port, and finally plugged in the HDMI cable between the adpater and monitor.  After that I downloaded and installed this firmware update.  With all that settled, I then simply uninstalled the following drivers: I did Windows Key + X > Apps and Features, scrolled down and uninstalled. Takes a few minutes and a few restarts, but I kept all my monitors plugged in, and during the graphics driver uninstall, my second external monitor sprang to life, and then flickered out, but that meant there was hope... Finally, I reinstalled the drivers. I did it through the Nvidia Experience app, which detected them all automatically and reinstalled, but I'm sure the same can be accomplished by going into Device Manager > Display Adapters, right-clicking Nvidia GeForce RTX (graphics card model), and clicking Update Drivers. So it turns out it is mostly software. I bought about eight different adapters and cables to make absolutely sure it wasn't a hardware issue, which Dell support swares up and down it is. But it's not. It's an Nvidia firmware issue.  After all that, I finally have two external displays set up and ready to rock! This is another article that may be useful for fixing USB-C to Displayport issues on a Dell G15 with Thunderbolt: Dell support thread explaining how to do everything 
Hi all - I have a G15 5511 with NVIDIA GeForce RTX 3060. It has a Thunderbolt 4 port with DisplayPort alt mode. I am trying to connect my Dell monitor S2421HDF via the Thunderbolt port. I have tried a few methods so far: None of these have worked so far. The monitor works fine when I connect it with a regular HDMI cable. It also works fine when I connect it to a different computer with a regular DisplayPort cable. Lastly, my wife's laptop is able to use the VisionTek USB-C to 2x HDMI adapter without any issue (it is a Dell but I'm not sure what model). When I connect a USB-C to the Thunderbolt port, the computer acknowledges that something has been connected (it makes the little sound) but my the monitor says there is no input signal detected. Any help would be greatly appreciated!  
G15 5511 Thunderbolt 4 with DisplayPort with alt mode