@Jnq 

Sorry for the trouble caused, we will do some troubleshooting first and try to fix it. 

1. What is the current network diagram?
2. What is the led status of the Deco P7 when you lost connection? How do you make it back to work after that?

3. At that time, with computer connected to the main Deco directly, can you access internet? Try to simplify the network diagram, in other word, connect the main Deco to the modem only and then connect your computer to the main Deco to verify whether it works well.

4. If still the same, please connect the computer to the modem, bypass the main Deco to test whether the network works. 
5. Besides, try to relocate the Deco units or swap the slave Deco as the main Deco to give it a go. Instruction as below: 

https://www.tp-link.com/support/faq/2400/

Thanks in advance, and have a nice day. 

@Kevin_Z 

My main deco is connected to the modem only and no other devices are connected to it. I'm using my p7 in router mode. In total, I have 4 deco p7 units. I located them in the same positions as my old m4 units. My modem is on the first floor in the living room, so is my main deco. Second deco is also on the first floor in the hallway which is beside the stairs to my second floor. Main deco and second deco have direct line of sight. Third deco is in my bedroom which is almost directly above the second deco. The last one is in my basement. When I loose wifi connection, all the LEDs are green which indicates they are online and other devices such as my smart TVs and my Amazon Alexa are still working fine. I did connect my PC directly to my main deco with an Ethernet cable and it works. When I do loose connection on my smart phone or my wife's phone and tablet, after a few seconds, it will reconnect. The wifi icon will come back on our phones and tablets. Some times it will happen several times then become stable. As I mentioned, the LEDs are all green and I have internet access on other devices. I also tried to turn off the fast roaming function because the deco app says some devices may not support it but still the same. My old m4 deco was able to automatically push my phones and tablets to the closest deco unit and with the fast roaming function turned on and I never lost connect for once. I have already tried to swap a slave deco to main deco but Still no change. All my deco units are not close to any electronic devices or metal objects. As I said above, they are in the same positions as my old m4s and my m4s didn't give me problems. This all started after the firmware update. By the way, all my decos are p7 units, I no longer have the M4 units. At last, if my phone is connected to the main deco first which is in my living room and I go down to my basement. My deco app will show that my phone is still connected to the living room deco and only one bar of signal, no 5GHz, 2.4GHz only. Even if when I start to stream full HD videos in 1080p, the unit still will not switch and the videos will be in 720p or 480p. When I try to manually switch to 1080p, it will take about 10 to 15 seconds to load the videos and play. So I have to switch off my phone's wifi and turn it on again to connect to the closest deco unit and after that everything is all fine.