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Dell Inspiron 5720 with A19 bios installed can not switch to external monitor

I have a Dell Inspiron 5720 with A19 BIOS update installed.
According to the latest owner manual page 44 to reset bios to default
downloads.dell.com/.../inspiron-17r-se-7720_owner's manual_en-us.pdf
I opened the laptop case and released BIOS battery for more than 10 minutes.
This suppose to reset bios to factory default. However, its behavior has changed.
When turn on laptop power, after press F2 key to go to BIOS display, I then press Fn+F1, it does not switch to external display any more.
Fn+F1 used to be able to switch display to external monitor on this laptop.
My laptop display is bad. Only a quarter is visible. I can see the laptop is up running in BIOS on the screen, but can not make any change in BIOS without seeing it on the screen.
Please help. Thanks a lot.
By the way, this is what I did before run into this problem.
I select enable UEFI boot. I also select enable secure boot (I guess this may be the cause of the problem).
But I think reset bios to default, which I did, should clear these settings I made.

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