I've been using Graphene OS for 10 months and occasionally (roughly once a week) when leaving my home wifi, the device will not connect to the network. Restarting the device is the only way to get it to connect to the cell network.
Turning airplane mode on and off doesn't help. I've manually killed apps trying to get the network to connect with no luck. I try something new every time it happens and I ultimately have to restart.
I keep hoping that one of the many updates would correct this issue, but it hasn't been fixed yet.