Got the same issue yesterday with Archer 3150 v2. The 2.4G network sometimes visible, sometimes not, but non of my devices can establish a connection even with open network (disabled security). Perfomed hard reset and tried a different firwares as well. Changing channels doesn't help. BTW, if 2.4 visible (but not connectable) network analyzer from my Galaxy S8 shows very good 2.4 signal. There are no any other routers close to my Archer.

@Danize I have the exact same problem.  Very frustrating.  ArcherC5400 v2.0  1.2.5 Build 20190411 rel.52981.  I had to point all my 2.4 devices to another router which is one step from leaving TP-Link all together.

I have Archer C20 v5 00000005 (0.9.1 0.1 v0001.0 Build 190213 Rel.65367n (5255))
When turning off ssid broadcasting at a frequency of 2.4 GHz, I can’t connect from one of my devices that works at this frequency. At the same time, at 5 GHz everything works fine!
If ssid broadcasting at 2.4 GHz is turned on, then everything works.
I can’t connect from the HP 4530s Windows 10 laptop (802.11bgn Realtek RTL8188CE Wi-Fi adapter) and the H96-mini TV box.
Rebooting the router and laptop does not help.
A laptop and a router are nearby.

@Danize 

Attempt the below, one at a time, to see which yields better result:

• Feel the router, if the router is hot or lots of heat coming out, use a fan to cool the router, a regular fan or USB fan is fine.
• Set the WiFi Transmit Power to low (if the setting is currently set to high), and see if this helps with the WiFi issue. When WiFi Transmit Power set to high, the router consumes more power, creates more heat, might cause instability.
• Review the Wireless setting, under 2.4GHz, adjust the Mode, b/g/n or b/g or n only (don't forget to click SAVE).
• Next, perform similarly with the 5GHz setting if the above did not help (change the 2.4GHz back to previous setting).
• Another option is to separate the 2.4GHz and 5GHz WiFi SSID along with different password to see if this helps. For example, SSID tplink1 - 2.4GHz and SSID tplink2 - 5GHz, different password for each.

Possibly lots of trial and error to troubleshoot. Additionally, review the firmware and see if it is the latest, might help with some of the issues.

@Eddy_Mora 

Attempt the below, one at a time, to see which yields better result:

• Feel the router, if the router is hot or lots of heat coming out, use a fan to cool the router, a regular fan or USB fan is fine.
• Set the WiFi Transmit Power to low (if the setting is currently set to high), and see if this helps with the WiFi issue. When WiFi Transmit Power set to high, the router consumes more power, creates more heat, might cause instability.
• Review the Wireless setting, under 2.4GHz, adjust the Mode, b/g/n or b/g or n only (don't forget to click SAVE).
• Next, perform similarly with the 5GHz setting if the above did not help (change the 2.4GHz back to previous setting).
• Another option is to separate the 2.4GHz and 5GHz WiFi SSID along with different password to see if this helps. For example, SSID tplink1 - 2.4GHz and SSID tplink2 - 5GHz, different password for each.

Possibly lots of trial and error to troubleshoot. Additionally, review the firmware and see if it is the latest, might help with some of the issues.

@Kevin_Z hi guys(users of tp link)

if you guys finding solution to the 2.4 ghz disconnect. Please stop looking for it. Tp link support would only ask questions back to you. Would not solve this annoying problem. Its a firmware problem. I saw posts from 2017 about the problem until this day there are still the same problem in 2020. 
 

from a archer c6 user whose 2.4 ghz band not working

@Adin 

this is bullshit to pay ove $300 for a worthless useless ax6000. I puchased mine a month and a half ago and have had nothing but problems. I want my money back. I am calling on Monday for a refund. My net gear router worked for 9 years without one issue before I bought this piece of crap. Everyday I drop 2.4gh. Then need to reboot. This is costing me work productivity. 

@Danize Anyone had any luck this problem i keep loosing my 2.4G every other day.

I got the same problem here. Not sure it's related to smart connect being activated but every now and then both 5ghz on my AC4000 work just fine but the 2.4GHZ goes down. The router and web interface show the 2.4 working but no device can connect to it. Any suggestions on how to fix it?