Do not format the card. The image must not be copied, need to be applied.
Now:
- Download the SD card formatter from the official SD association: https://www.sdcard.org/downloads/formatter/
- Install it and format your card, using the "Overwrite Format". At least wait around 30 min. and cancel the format.
- using winrar or any program that can extract gz files, decompress the "rpi4.64-snapshot-23290-1.9.15-29-r14315-ext4-fac.img.gz" you downloaded, the resulting file should be "rpi4.64-snapshot-23290-1.9.15-29-r14315-ext4-fac.img"
- Use Win32DiskImager from here: https://sourceforge.net/projects/win32diskimager/
- Select the image file, the sd card device, hash=none and press the "write" button. Do not bother trying to verify it, it will fail most of the time, since you already wrote something on the card.
At this point the card should be ready to put in the raspberry, do it. Connect the ethernet cable to the rpi native thernet port, connect the dongle to an usb3 port, and connect the dongle to the modem using another ethernet port. Wait one or two minutes, and launch your web browser to https://192.168.1.1. Depending on what browser or OS you're using, it will throw an certificate error. On firefox (other browsers you need to google "Allow self signed certificates" plus the name of your browser) you need to press the "advanced" button that will appear, then "add exception" then mark "permanently store this exception" and finally "confirm security exception". Then you'll get the welcome screen of openwrt.
That should do it. Far easier doing that write it.