Hi RFinPRC,
Welcome to Microsoft Q&A forum. I'm happy to help.
Please refer to this official ASUS article to setup your Router: [Wireless Router] How to set up Media Services and Servers- Network Place (Samba) Share/ Cloud Disk?.
Please notice that in Step 4, please don't enable Allow guest login because from Windows 11 24H2 - as a security enhancement - you cannot access shared location as guest, therefore Windows will deny such action. So, it's better to access SMB shared location with User name and Password.
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Please also notice that if you cannot access shared location with their name \\RT-BE386U, you may use its local IP instead, example \\192.168.1.1 - using IP is also more reliable.
You also may need to clear cache credentials by open Credential Manager > Windows Credentials > look for credentials that related to SMB under Windows Credentials section > Remove.
I hope that above information is helpful to you.
If you have further questions, please feel free to let me know.
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