Ok that's good.
Have a look here
You should be able to start a telnet session by just using the browser. you won't get any specific page just an error but the you can try and telnet to the router with admin and your admin password.
If that works it would be better to try and get a full backup of the device. Does the device have usb by the way? From the pictures it seems like it does not. Am I right?
Now the disclaimer
I HAVE COMPILED THIS FIRMWARE WITHOUT MUCH INFO FROM YOUR DEVICE. I HAVE ASSUMED THAT IT IS USING THE SAME PARTITION SCHEME AS THE DIR-878 SO THIS IS WHAT i USED.
THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE
Ok now that's done this is the file. Try and flash it from that web interface. It does not mean it will work the first try.
If it does not I had some success with windows edge in private mode. If that does not work try with curl
curl -v -i -F "firmware=@openwrt-ramips-mt7621-dlink_dir-853-a2-squashfs-factory.bin" 192.168.0.1
This is the file. Please understand that it might brick your router.