I recently installed KDE neon and found that my network manager is not able to detect my wireless networks.
I am a beginner so this maybe a noob question.

here are some of the outputs of the codes i ran :

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lspci -nn -d 14e4:
04:00.0 Network controller [0280]: Broadcom Corporation BCM4312 802.11b/g LP-PHY [14e4:4315] (rev 01)
07:00.0 Ethernet controller [0200]: Broadcom Corporation NetLink BCM5906M Fast Ethernet PCI Express [14e4:1713] (rev 02)


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iwconfig wlan0
wlan0     No such device


Thankyou in advance

Then name of your WiFi might not be "wlan0", hence the reason why you're getting "wlan0: no such device".

Run this:

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sudo lshw -C network | grep -i 'logical name'

On my system, this returns:

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logical name: enp2s0f1
logical name: wlp3s0

So in my case, the name of the wireless connection is "wlp3s0". In your case, if nothing is returned, it means your WiFi interface hasn't been brought up (which could be due to a driver problem, or a number of other things).

If you have an Ethernet connection, this worked for me:

Open a terminal

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sudo apt-get purge bcmwl-kernel-source

If you don't have anything to purge, that's ok.

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sudo apt update

then

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sudo update-pciids

If nothing is updated, no worries.

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sudo apt install firmware-b43-installer

after that's done:

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sudo reboot