How to lock up a bike with a rear wheel lock?
Living in a country where this is the standard bike lock I have learned to work with it.

For a short stop where the bike is at little risk or while getting your second lock out, always lock this lock as soon as stopped.
Then you get a second lock out, U-lock, a cable or chain as per your choice or your insurance requirements or a combination of those.

As the rear wheel is locked to the frame, you can now select to use your second lock for the front wheel to the frame and both to a fixed point.
Or you can get a cable to be locked into your ring lock, either as a separate entrance to the lock for which the lock has its special hole, or one with a loop to go round the locking pin. And use that to fix the bike to a fixed point when you chose to lock the front wheel to just the frame.

I have always chosen a separate cable lock (when insurance allowed) or a chain to connect the frame to a fixed point, but my bike is not as valuable and always one of many where parked.
For a valuable bike I would likely use a good quality U-lock, a good quality fixed point and use that to lock the front wheel and frame.
If no good quality fixed point is available, I would not leave the bike there.