The best way to achieve your solution would be by using a Traffic Rule. The rules can be configured by navigating to Network → Firewall → Traffic Rules.
The first rule will be used to allow the wanted traffic to pass through:
- Select Open ports on the router;
- Create any name;
- For protocol, select Other;
- For port, enter 0 (it will be disregarded since the protocol is selected as any);
- Press Add;
- When the advanced configuration window opens up, make sure that the Source zone is set to lan;
- Destination address is the IP address of the traffic you'd like to allow through (multiple addresses can be added);
- Destination zone needs to be set to wan;
- Make sure that the Action is Accept;
- Press Save & Apply;
Now a rule blocking everything else is needed. I won't rewrite all of the steps, but what will be different is:
- Source IP address: left empty;
- Source zone: lan;
- Destination zone: wan;
- Action: Drop or Reject (Reject will notify the user that the requests were blocked, while Drop will drop them silently);
And that should be it!
The router may need to be restarted in order for the changes to take effect.