@Mardan100
Hi,
Please try to turn off 2.4ghz, and leave 5ghz only on the main network.
Did you have a slow wireless speed on different clients from Deco E4?
Please also refer to this link to help me check the wireless link speed on the mobile devices:
How to check the associated/link speed on a wireless client
If unplugging Deco E4 temporarily, stand where it is and connected to the main Deco M5. How much speed you could get on the device now?
Wait for your reply.
best regards.