Hey,
please permit my novice question. Am new to wireless networking and mikrotic.
Am extending our LAN to connect a new office. the 2 offices are separated by a distance of 5km with an unobstructed LoS.
i have installed the mikrotik RB711-5Hn at both sites but the signal level is too low. see attachment.
I have been advised to change the frequency. Attached is the frequency usage output.
My question is from the output attached how do I determine a frequency that is free that I can use for the link.

Your main problem is that the signal is very low. Most frequencies seem clear.
The distance is not big, but the signal will depend on the type of antenna used and you didn't mention your antennas at all.