snapshots are built daily, and that sets time limits to installing new packages with opkg. Due to kernel version checksums, you can only install “kmod” kernel modules and other kernel version dependent modules from the exactly same snapshot build. So, a few hours after flashing the firmware you may not be able to install new modules with opkg any more (as the next snapshot has been built into the download repo and has different checksums).
Download the new snapshot and use sysupgrade to flash it.
If your model has a release build, that is usually suitable for the regular user. A matching permanent package collection is kept so that the problem of uninstallable packages does not occur.