Gran Turismo 7, it seems, follows in the fine traditions of GT Sport, GT6, GT5, and even GT5P in being a bit sensitive to network environments. A connection that's fine one day (or hour) might be ropey the next, and while there's a lot of things that might be out of your control, the best way to improve your experience is to make sure the bits that you can control are as good as they can be.
You don't need much technical knowledge at all to optimise your own network environment and make it as stable and friendly as possible. Here's a quick guide to doing so which will resolve many...
Obviously if you have to be on wifi, ignore the bit about wiring it it.