@pot
Hi, welcome to the community.
I suppose the problem might be on the Ethernet link speed from the WAN port of M9 plus to the ISP modem.
Could you please help me check some details:
1. What is the speedtest under "Test internet speed" under Deco APP>More?
2. please connect the ISP modem to another port on the M9 plus and if possible, please change a different cable as well.
If still the same, you could also change a different Deco M9 plus as the main Deco:
How to Setup the Satellite Deco to be the Main Deco?
Thank you very much.
Best regards.
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