@Ribeiros
Hi, welcome to the community.
Since both Archer C5400 and Deco X50 could work as wireless routers, did you turn Wi-Fi off on C5400 and use the network of Deco X50 only?
I noticed you mentioned that "2 units of a Deco x50 connected to my router tp link c5400 via wired connection".Since when Deco X50 is on wireless router mode, only the main Deco could connect to the C5400 via Ethernet cable. Did you also connect the satellite x50 to C5400 or behind the main X50?
As for the firmware downgrade, if the original firmware is 1.2.2, the new firmware 1.2.5 barely changed anything compared to 1.2.2. I don't think it would cause the slow speed.
If you want to downgrade the firmware, it would be accomplished via web UI directly and the process is the same as the firmware update.
How to Update the Firmware of Deco(method 2 via web UI)
firmware link for 1.2.2:https://www.tp-link.com/en/support/download/deco-x50/v1/#Firmware
But If possible, I hope you could check whether the following suggestion could help fix the current issue:
1. replace a new cable to connect the main Deco X50 to a different LAN port on C5400.
2. log into the web UI of C5400 and force all the LAN ports to 1000m/s full duplex mode.
![](https://static-community.tp-link.com/other/30/5/2023/e93773066fcf4c5d80fa4189d5046d20.png)
Thank you very much,
best regards.
Wait for your reply.