What is the surge protection in modems. whether

This video shows it well. You need to balance loads but yet, one neutral is in the center, How the 3 phase math works 277 277 480

A spike is an increase in voltage for 1-2 nanoseconds.
A surge is the increase lasts 3+ nanoseconds.

Need more info. What are the measurements on the input to the UPS. What is the total load you have? What other alarms are displaying? Check the input power to UPS. Also make sure the UPS is and will work for the correct voltage (Input of UPS) some UPS's are selectable voltages.

Yes, the surge protection works on both the switched and unswitched outlets

something is loose or power not correct. ck polarity at outlet. narrow pin is hot wide is neutral and round center is gnd. 120v hot to neu and gnd 0v neu to gnd

I happen to have one of these in the box, so I've reviewed the installation & on-line literature for this model. Let's address installation notes:
Don't connect more than 250 ft of load conductor for the total one-way run, to prevent nuisance tripping.
The breaker is to be used on grounded power supply circuits only. We're talking a properly-grounded breaker box, not just the protected circuit.
Look at the side of the breaker. You will notice that the curly white wire is meant to be connected to the (properly grounded) neutral bar in your panel.
The circuit neutral that you're protecting should be landed on the terminal just above that curly wire. Make sure you have the right neutral!
The circuit's hot wire would of course be landed on the topmost screw.
You did not state what you mean by "the breaker next to it": just above, just below, or directly across from the GFI breaker?
I suspect that you meant just above or below the GFI breaker. And I assume you've swapped out other breakers to rule out a defective breaker.
Now, it is possible that you have a "shared neutral" situation. It's a common wiring practice to use one neutral wire for two "hots", where one circuit is fed from the phase A side and the other is fed from the phase B side, (which you'll have in a two-pole, 220V breaker), picking up a 110 volts from each phase. The two 110v "Hots" share a single neutral wire between them to carry return current. The phase shift between the two phases allows this.
However, to avoid nuisance tripping of your GFI, your protected circuit cannot share neutrals with another circuit, as the "other" circuit's operation will cause the 6 milliamp differential between current out (hot) and current return (neutral) which the GFI by design senses and trips.
Your GFI-protected circuit probably needs its own dedicated neutral!
I'd like to hear what you find. Good luck!

The powerline modems are affected by the other connected equipment in your house and the wiring phases.

For BOTH modems try the following first:

-Connect the modems directly to the wall socket, don't use an extension lead.

-If you have a surge protecting extension block plugged in in the next/adjacent socket, try plugging it in on a different mains socket.

If this does not work, you may want to think again about routing an Ethernet cable between floors.
Higher performance and lower cost.

1. Why not buy a phase loss monitor that monitors all 3 legs. 208v 3ph is 115-120V to neutral. If you're using two 120V relays you'd only be monitoring 2 legs of the 3 phase. Using relays can be risky, as sometimes when you loose one phase you'll still pick up voltage through your load. If your set on using relays use 2-208V relays. Connect one to phase A+B and the other to B+A and then wire two of the normally open contacts in series and put that in your circuit with your pressure switch. I'f you're using 208V for control voltage, then you need to take L2 to the oveloads and then to the coil. L1 would go to your pressure switch, and the return from the pressure switch would go to the other side of your coil. You could add your phase loss protection here.

it requires 3 phase with ground it is a four wire system

open ground=ground wire (green) is not connected to the circuit box ground or to the earth ground.
(there should have steel rod place in the ground by the local electric company, and gtound wire is connected to that rod).
Open Neutral= same as open ground. unless you are testing 240V, which have 2 hot (110v) and a neutral (0V).
Open Hot= open 110v line (black)
hot/ground reversed= if you understand the above, then this sould be self explaintory.
same a hot/neu. reverse.

if you don't understand how electric wiring works, My suggestion is to stay out.