When using the OPC UA Client app, it is necessary to start the connection manually as of version 1.16.0.
This is detailed in this post:
In this same post, a Node Red example is given for starting the OPC UA Client if the current state is Disconnected. This can be used on startup or to recover if the connection is lost during runtime.
Attached is PLC Code to do the same. The file is a PLCOpenXML file that can be imported into the ctrlX PLC Engineering program.