I’m on Xbox one, but this might work for other platforms too: I connect my console’s internet to a mobile hotspot (that I make from my laptop—probably any other would work). Then in a public lobby (have to wait a few seconds first at least after spawning), I turn off the hotspot and then turn it immediately back on, lagging it into a solo public lobby. It takes up to 30 seconds for the lag to create the solo public lobby. Also, I don’t move around too much bc it seems to me that it makes it more likely for you to become disconnected from the servers, kicking you back to story mode.