@TP-Link 

Thank you so much for the rescue and I'll definitely try the link you sent right away.  Really appreciated.

By the way, I'd assume that doing the firmware upgrade wirelesly, as long as the connection is ok, wouldn't have any impact, would it?

I did it wirelessly yesterday using the the firmware dowloaded from the TPLink website, then rejected by the machine (several times).  Nothing wrong about the connection came up, and only the rejection message popped up during the upgrade which went as far as 25%, and then it went stone cold, fizzled.

Wish me good luck, and really really appreciate your time.

@TP-Link 

This is a so hitting-the-nail-on-the-head question.  I'll set a little context first below so that you can follow, and then an anectdoe at the end about my experience of using this line of TPLink product.

1. Got this unit as  a replacement in Dec. 2017 from TPlink because they couldn't fix the old one I sent them for repair. 

2. Since Dec. 2017, I rarely used it because I was assigned to projects overseas. So it was put away until I came back on annual leave to use it, meaning only few times.

3. Just came back few days ago, currently under quarantine at home in isolation.  My family prepared the new SIM card for me before they left.

I plug in the SIM card and powered it on, working as a charm.  But within less than 10 hours, it just went dead: Laptop showed diconnect, but unit display showed normal: false pretense.  Then, I powered it back on, the unit display went blackout during "initializing."  I tried "RESET" with no luck, repetitively.  But something interesting I found after try and error: when the SIM card is removed, the unit display came alive after I pressed the power on and stayed that way.  If reinserting the SIM card and followed by pressing the power, the same blackout scene happened again (keep pressing the button wouldn't bring it back; need to wait for about 30 seconds, but it still went blackout during "initializing").  

That's where I thought maybe the firmware needs an update.  I also talked to TPlink customer service and she said I could give it a try.  And the sad thing is my unit's warranty expired already, and they can't do anything about it even if I am willing to pay for the repair: she told me I need to buy a new one.  So that's the whole story....

Now, here is the anecdote for my experience with this line of TPLink product:

My first purchass of TPlink Mobile WIFI router was in Jan. 2016, worked just fine.  Then, I got assigned overseas in Aug. 2017, so I put it away, until I came back for a break in Dec. 2017: it was dead.  So here you can now re-trace back to No.1 above for a full picture.  So my verdict about this product is this: if you put it away for more than 6 months, chances are it would probably die on you when powered on after that storage time.  I tried two already.....  ¯\_(ツ)_/¯   

As usual, I am grateful for any suggestion you might have.  Thanks a million.