Thanks @October31,

The new firmware of Archer BE550 is 1.1.2 Build 20250317, Enhance Your Experience: Archer BE550's New Pre-Release Firmware Boosts Stability and Performance! 
Also, please make sure that the firmware version of RE705X is the latest version.

And try the following suggestions:

• Could you check that have you set up the easymesh network correctly? The RE705X should be under Mesh list.
• For RE705X, please move the placement to shorten the distance between it and the router.
• Is the Wi-Fi mode the same? If not, please try to change to the same Wi-Fi mode. Go to Advanced--Wireless--Wireless settings--Mode.
• Please set 2.4G and 5G SSID separately, and compare which band the problem occurs on?