@JoSW 

Thanks for your information. Hope you and your families stay well.

Back to your case, we can do some troubleshooting first to locate this issue and try to fix it. 

1. What is the current network diagram of Deco M5? And what is the led status of Deco M5 when it lost connection. 

2. Do both wired connections and wireless connections stop working at the same time? Try to connect the computer to the modem directly, bypass the Deco and verify whether you can access internet. 

3. You can swap the locations of the Deco units; or you can switch the slave Deco to the main one to give it a go. 

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

Good day.