Or, which also sometimes happens, it is returning an endpoint with an IP address or a host name that is not reachable by the client (e.g. a local network address while the client is on the public Internet). Or an address prevented by firewall.

Yes, you can simply specify a numerical IP address in place of the <computer name>. I would say that you should, however, first try to resolve the actual underlying problem of a changing IP address which probably shouldn't be changing at all.

Be aware, however, that the OPC UA connection process actually works a bit differently from what the intuition tells you. The process actually has two phases:

1. The discovery: The client connects to a so-called discovery endpoint of the server. For this, the client will use the address you specify (either numerical, or a symbolic name with DNS lookup or other available lookup mechanism). The server responds with a list of endpoints (which may differ in transport used, security, etc.).

2. The client picks an endpoint from the list provided by the server, and opens a channel to the server. In this second step, the client can use the IP address or host name that is specified BY THE SERVER in the list of endpoints - not necessarily the same as the IP/host name you have originally specified.

The reason why opening the channel fails due to changing IP address may therefore actually be on the server side as well.

Also note that there are settings in QuickOPC that can be used to enforce the usage of the same IP/host you have specified originally (although, sadly, that brings problems in other scenarios...). This alternative setting should be considered if using the numerical IP does not resolve the issue and you would still see a different IP in the error message.

Best regards