Hello DrunkRabbit!
MAC address filtering on wired networks:
(1) Click Advanced Configuration > Security Configuration tab, and choose MAC Address Filtering in the navigation tree on the left. In the pane on the right, after enabling MAC filter and selecting the filter mode, click New. On the dialog box that is displayed, configure the MAC filter rule for the PC to access the Internet.

(2)Click Apply.
The MAC address lists of PCs in the network are saved on the ONT. Configuring MAC filter rules enables the PCs that conform to the rules to access the Internet service or disables the PCs that do not conform to the rules to access the Internet service. A PC may have more than one IP addresses but a unique MAC address. Therefore, configuring MAC filter rules effectively controls the Internet service access rights of PCs in a LAN.
Attention please:
The above is the web setting method for the general version of the corporate network. If your device was obtained from an ISP, please follow the steps below:
1. Access method is LAN port
In the navigation tree, choose Home Network > LAN Devices > LAN Devices Settings. In the pane on the right, you can view and configure the parameters related to the LAN devices.

You can click Edit to edit the related LAN device.
You can click New to add a new LAN device.
You can click Edit forwarding rules to edit the port forwarding rules.
You can click Edit DMZ to edit the DMZ rule.
2. Access method is WLAN
In the navigation tree, choose Home Network > WLAN Access > WLAN Access Rules. In the pane on the right, you can permit or prevent a specified wireless client from accessing the WLAN, then click Save.

Allow all computers in the WLAN: After Allow all computers in the WLAN is selected, all computers are allowed to access the WLAN.
Only allow specific computers in the WLAN: After Only allow specific computers in the WLAN and the computer name are selected, only the selected computer is allowed to access the WLAN.
Disallow specific computers in the WLAN: After Disallow specific computers in the WLAN and the computer name are selected, the selected computer cannot access the WLAN.
If none of the above solutions achieve your purpose, it is possible that your device is a customized device and some functions have been restricted. Please contact your device provider.
Thank you!