Thanks for your reply. I have tried this method but the "Advanced Boot option" is not showing on my laptop. It seems that this is a simplified/shortened version of bios.  However, I was able to reinstall Windows in UEFI mode. Now the system starts normally except that the bios settings including secure boot cannot be changed back to default.   
I have posted a related question under SupportAssist session: [URL]#M94892 I'm posting the question here with update thinking it is more related to Bios settings.  Background: the pc came with a 250 GB ssd (HD0) and Win10 home. I added a 2TB ssd (HD1) and installed Win10 pro to this HD1, removed the Win home, later upgraded to Win 11. Recently the touchpad had no response so I reinstalled Win 10 pro thinking a Win11 upgrade was the cause, but this didn't fix the touchpad problem. Then I came to dell.com updated the drives and installed Dell's SupportAssist and a new problem appeared.  Now "no bootable device" shows on startup. Below is the list of settings in bios and the things I have tried according to the info I learned: Any suggestions are greatly appreciated!     
Inspiron 5505 no bootable device