Hi @diegosucaria,
I was getting same problems as you, and then set up the connections in boot partition. Even with the connections set in that folder, and disabling the resin-wifi-connect script from start script, I’m managing to get the same disconnection errors after 5 or 6 hours. What could it be?
EDIT:The problem was on wifi signal from customer. After it normalized, the Network Manager managed (pun intended) to reconnect automatically. The system is now working fine.