@NIK7 

Hi, welcome to the community.

Here are some suggestions for your reference:

1. make sure the QOS has been disabled.

2. turn off "UPNP" under Deco APP>More>Advanced>NAT forwarding.

3. connect fiber ONT to another Gigabit Port.

If nothing helped, please send an email to support.forum@tp-link.com Subject-Forum ID 614456 Deco XE75 Pro  with the following information:

1. May I know who is your Fibre service provider?

2. Can I have some screenshots of the speed test result when the devices are connected to the main Deco?

• Please also refer to this link to help me check the link speed on the tested device.

Thank you very much.

Best regards.