Thanks for the very clear shots.. helps no end.
Does that just signify that it is a wireless connection
Yes.. solid for ethernet.. dotted (sort of ethereal for wireless)
I'm concerned that it says "connection Unknown".
That is just the latest update bug.. instead of the previous problematic version.. it now always has connection unknown.. except that this is patently untrue .. simply roll your mouse pointer over the "unknown" to get the actual link info. This is actually useful. A little popup will appear telling you the actual data rate, signal and band.
My example is done very close.. and is running at full speed and signal.

I took the screen shot after the connection, and it looks like the printer connected to the second Airport, which is much farther away than the first (??).
Distance without taking into account walls and other hinderances in the signal path can be confusing.
Unfortunately you cannot force the connection to the best device.. as the client will work out for itself best.. which may or may not be appropriate..
BTW you can do the same thing for the client as you can for the router to router connection.. roll your mouse pointer over the connection and a whole lot of actual numbers will pop up. This will help you keep track of what is happening.
To force the connection back to the main AE.. you can power off the extender for a few minutes.. maybe need to also power cycle the printer.. it will swap to the closer AE.. and you can then inspect the numbers.. I would expect the signal strength from the extender is better than the main one.. as the client will always join to the best signal.

I heard someone say that I should toggle wifi on and off on my devices now and then to make sure that I am connected to the nearest AirPort and not locked onto one that may now be further away. Is that correct?
For most items this is true but unnecessary unless you are really having signal issues. Or speeds are very poor.
Perversely the items you would hope to roam well, iPhone, iPad etc.. tend to behave poorly.. and items that you carry around less.. Macs and printers etc.. work better.. in fact the Mac laptops work really well.
Wireless is still at the voodoo stage of development.. what is unscientific is best handled with beads and rattles. 😝 Perhaps with a bit of old wives' tales and snake oil thrown in for good measure.😉
Engineering wise it probably gives 50% improvement.. i.e. it is luck whether the ball lands on red or black. Over long periods the statistics prove it is random.
And I do feel the new AirPort is extending the signal since my speed has increased in some of the distant locations in our house.
Actual speed is the real test.. this is not the best way to extend the network.. it is however by far the cheapest and fastest and easiest. If future use shows results below expectations.. then wiring ethernet between the routers will give you a far superior network. Certainly if you own the house.. wiring for data is recommended. Long term it works.. fast and reliably.. ie opposite to wireless.