Please go to https://kc.dlink.com/Home/FindBlob?FileName=dir822
You will see three versions of your router. So please take another look on your router. You will find a label with the serial number, the type and the release A1, B1 or C1. Download the right manual for your router. Okay, I could only load the A1 manual and the others seem to be damaged.
First of all you need to find the correct ethernet ports. On page 5 of the router manual you see the router backside. You must connect the yellow port with an ethernet cable to the internet (this is your school network or a connection to your normal internet router).
Then you must connect one of the grey ethernet ports with a second ethernet cable to the MoodleBox.
You should start the router. It needs two or three minutes to start up. Then power on your MoodleBox. When you connect your personal device (smartphone, tablet, notebook) to the MoodleBox Wifi then you will get the frontpage http://moodlebox.home/ and you can login as the main user moodlebox.
Please go to the MoodleBox dashboard http://moodlebox.home/admin/tool/moodlebox/ to see the ip addresses. You will find the cable based interface eth0 and its ip address. If the MoodleBox got a cable based ip address then everything works fine. Write down this ip address. You will find this later in the router configuration.
Now you can also try to connect your personal device to the internet. Open the web site https://moodlebox.net/ ... if the page opens also the internet connection is fine.
Now connect you personal device to the DIR-822 router. You can do this via cable to one of the the free gray ports or via Wifi. Read the manual how to do this. If you are connected please do not start the "Internet Connection Setup Wizard". Your router should not take parameters and passwords for your internet connection. It should connect to the main router and the MoodleBox should connect to the DIR-822.
On page 53 you will find the network setting for your DIR-822. Your DIR-822 needs to have a static ip address which differs from the main router. If the main router address is 192.168.0.1 then your DIR-822 needs to change the default address to 192.168.1.1 or 192.168.2.1. Take in mind that no internet connection will work if both routers have the same static address. Also the host name should differ in both routers. Save your settings for page 53.
On page 54 you see the address range for the DIR-822. As the default the range is from 100 to 199 ... this are 100 devices but I can't say if all 100 can connect at the same time.
On page 56 you will find the dhcp reservation. Click on enable. It could be that you will find the "moodlebox" in the dropdown menu behind the computer name so you can choose an existing one. The computer name must be "moodlebox". Take its MAC address from the table two places under your entry field where you add a new dhcp reservation. The ip address can be a new one ... perhaps 100 for the last place so you can better remember the ip address ... your MoodleBox will get the ip address 192.168.0.100 if your DIR-822 has the static address 192.168.0.1. But your dhcp reservation must be within the dhcp range you see on page 54. Yes, you can change the range for the last place from 2 up to 254 ... these is the maximal address range but I don't believe that the DIR-822 will offer Wifi for 253 devices.
When you now save the settings and restart the router the ip address 192.168.0.100 is reserved exclusive for the MoodleBox. And the computer name "moodlebox" should also be reserved for the MoodleBox. If not, please look at the MAC address. Is it correct?
Try to connect the url http://moodlebox.home from your personal device connected to the DIR-822. Does it work? Yes now you are ready. 25 devices can connect to the MoodleBox Wifi and the others can connect to the Wifi of the DIR-822. No, please don't use the same SSID for the different Wifi.