You didn't mention any information on your computer and Monitor and how it is connected to the GPU. So I will just be guessing in my next replies.
First see if by updating the AMD driver you are able to get a higher resolution again from here: https://www.amd.com/en/support/graphics/amd-radeon-r5-series/amd-radeon-r5-300-series/amd-radeon-r5-...
Even though the above driver is for the R5 340 AMD GPU card it should work for your AMD OEM GPU Card which is based on the R5 340 GPU card.
If that doesn't work then make sure that in Windows Settings - Display, Radeon Settings, & on your monitor you have every configured for the higher Resolution.
Otherwise since it had a higher resolution with the old BIOS Chip that was replaced by a new BIOS Chip it seems like the problem is with your BIOS.
It is possible when they replaced the old BIOS chip with the new BIOS Chip, the new BIOS Chip has an older BIOS Version installed.
So see if you can update the BIOS version to the latest for your motherboard from BIOSTAR's Support Motherboard download site.