Hello,
In my case, the stock FW had wget which means you can download anything directly to the router. If you connect the router in a way that it has internet access, then you can probably even do:
cd /tmp
wget https://downloads.openwrt.org/snapshots/targets/ramips/mt7621/openwrt-ramips-mt7621-dlink_dir-2660-a1-squashfs-factory.bin
Note: editing the link above - in the first version I had pasted the wrong image link - the initramfs-kernel instead of the factory one.
(but do confirm the image sha256sum before flashing it. I also connected a usb flash drive and saved on it all the mtd partitions before flashing anything. I'm not sure how exactly I would use this backup if things would go wrong, but better having it than not)
If you would like to download the image from your local network, starting a temporary http server on another system that listens to <port number> is as simple as running:
python -m SimpleHTTPServer <port number>
in the directory you want to share (the directory on the other system that has the firmware images). This should run on almost any linux distro with python 3.