One thing that could be done, is a TLS client for each of these devices could be set up, thus static IP would be assigned to them, as at the moment you do not have any TLS clients, and IPs get assigned randomly.
TLS clients can be set up in the OpenVPN configuration, and you'll also need to sign new certificates for the other clients. If you generated certificates on the device itself, then new certificates can be signed by navigating to System → Administration → Certificates. Under file type, select Client, Key size with the default certificates is 2048, and make sure CN is unique from the other clients. Select the Sign The Certificate option and enter the number of days the certificate should be valid.
Your Windows logs indicate that the connection is successful, so I believe the issue could be with the IP assignment.