Hey there! @John_williams2, Welcome to the Community! 
Thank you for the tag @JDRoberts! 
I have multiple suggestions that may assist you and others.
Make sure you are running the latest version of Bixby Basic Commands, this can be found in the Galaxy App Store and ensure the best functionality of Commands. V 1.4.05.5 (Updated June 3rd, 2021).
Double-check your Smart Things Capsule, this is where Smart Things Devices are linked to the Bixby service. Note: This is automatically linked to Bixby on Samsung Devices and newly onboarded devices will automatically appear, and the capsule cannot be removed.
Check your Device naming system that has been linked to Bixby by your SmartThings Capsule. Device Type for your Meross WiFi Device would be a “Switch”, the name of the switch (Fan, Heater, etc). This includes duplicates for example two fans are listed for the “Turn my fan off” command, If you have a Fan 1 and Fan 2., the devices will populate on the screen and ask for your selection of which fan. Bixby would be looking for the command for device type being a Switch or the Device Name Fan(Not Including the Room device is located in).
To include Devices by Room in the Bixby Settings Menu, You would need to set up a Custom Quick Command for ‘What you say to Bixby’ “Turn off Bedroom Fans”, you would then need to add custom “What Bixby Does” commands. Select a Command>Select Smart Things>Select a pre-set command or Type your specific command that triggers that device. “Turn my fans off”. Additionally, with the new Grouping Feature of the Smart Things App, Bedroom Fan 1 and Bedroom Fan 2 are able to be grouped as “My Fans”. Groups are detected by the Smart Things Capsule allowing for the Group to have On/off Functionality.
The best way to achieve this and get familiar with Bixby commands is to start by asking/activating Bixby “Show me my” followed by these commands separately “Smart Things Devices”, This will list the number and name of devices that are able to be controlled by Bixby. Using this information and your naming convention to establish what to say to trigger the Meross Device’s On/Off Features.
Additionally, things that may assist you or others: “Show Me My Switches”, and “Show Me my Automations”.
I hope this message finds you well!
^SamsungZell