go their global website to search EU version since yours is TL-WR840N(EU)V5

https://www.tp-link.com/en/download/TL-WR840N_V5.html#Firmware

Excellent! Thank you very much for your answer. I will use the firmware shown on that page.

EDIT: I was able to update the firmware with the file that you had pointed out. Thanks! again. I would like to point out an important fact that might help others who are having a similar issue. The (EU) only appeared in the name of the device printed above the S/N on the back of the device, and I think that EU combined with the assistance from Bowen98 got things rolling in the right direction.