Before downgrading to version ELS-LGRP4-OVS 10.1.0.131, I CAN update OTA.Doing some research I have found what is happening:
On your P40 Pro, in the "About phone" screen you can see the build number and then a number between ( ), example: Build number 10.1.0.131 (C636E8R5P1)
The number after the C will tell you the version or region of your firmware, in the example C636 = version/region 636
When you downgrade using a different version/region you will loose the OTA updates on the phone, and this will only be restored after you install your correct version/region firmware.
You will need to find the correct version of your firmware and have the "dload" version, and install it from external card. There is a video showing how to do this, but the link on the video is for a version/region 636, if you have a different version/region it won't fix anything.
Once you have the correct firmware and install it, you will begin receiving OTA again. This process, at least on my phone, erased everything, so be aware of this.
I downgraded to version ELS-LGRP4-OVS 10.1.0.131 then update to ELS-LGRP4-OVS 10.1.0.134 many times, BUT I cannot get an update OTA.
My P40 Pro current version: 10.1.0.134(C792E7R3P1)
Model ELS-N04
How to download firmware with the region C792.
Cust Software Version: ELS-N04-CUST 10.1.0.7(C792)
Preload Software Version: ELS-N04-PRELOAD 10.1.0.1(C792R3)