Corsair Dominator Platinum RGB with the MANUFACTURER number CMT16GX4M2Z3200C16.
is not mentioned on the ASUS Prime Z390-A list.
https://dlcdnets.asus.com/pub...
Nevertheless, you have managed to get the device working with a latch in position A2.
According to the list above (notes at the bottom: 1 DIMM), you can insert a bolt in one of the four positions (A2 / A1 /B2 / B1).

To make it easier for you to track the results, it is advisable to create an Excel spreadsheet, for example (do not connect to the Digitec network):
First column: Code for PrintScreens of the settings (as you already have one connected) (e.g.: "Cor-eins-A2-21-01-a" means Corsair, 1 bar in position A2, 2133 MHz, attempt 01 Screen a).
Second column: MHz (e.g.: 2133)
Third column: Volt (e.g. 1.2 (one POINT two), depending on the frequency)
Other columns: the four times (SPD Latency 15-15-15-36).
Last column: Notes
These values (MHz, V and Latencies) must be entered / checked for each position in the BIOS.
https://www.corsair.com/us...
Make a "PrintScreen / ScreenShot" of each setting and save it on a USB stick (do not connect to the Digitec network). This way you can check at any time under which conditions the unit worked well. Are the newly entered values still present in the BIOS when you restart?

It's a bit of hard work. Check all four positions under the different frequencies WITHOUT overclocking, i.e. to a maximum of 2666MHz and a maximum of 1.2V, see
https://www.asus.com/de...

Questions:
Will the PC boot with one latch in each of the four positions up to a maximum of 2666MHz?
Does the PC boot if the positions A2 and B2 are first set in the BIOS and the second bar is only inserted afterwards?
What experiences have been made with the Corsair kit CMT16GX4M2Z3200C16 in combination with ASUS Prime Z390-A (without OverClocking) (ask Corsair Support or Community (Corsair kit is not listed)?

With a bolt in position A2, as the PC is now installed (functioning initial position):
Save the BIOS including voltage and frequency (all together called "values") as a document (if necessary by means of a print screen) so that the initial situation is known and can be looked up if necessary.

Set voltage and frequency with 1 bar, starting with 2133 MHz. Save the "values" as a document.
See what happens at 2133 MHz with one bar and then inserting the second bar (positions A2 and B2 occupied).

So 2 essential points are already okay: each Ram bar works on its own, and the two bars were bought in a kit.

Were the two bars inserted according to the brief instructions: A2 and B2 occupied?
https://dlcdnets.asus.com/pub...

With the appropriate voltage setting (usually 1.2V, see Corsair instructions for details) and in the combination of A2 with B2:
What happens at 2133MHz / 2400MHz /2666MHz (no OverClocking)?

Perhaps 2 RAM bars bought separately, i.e. 2 kits of one bar each(?).
Therefore the following questions:
RAM: Manufacturer, product name, MANUFACTURER number.
Processor: Manufacturer, product name, MANUFACTURER number
Power supply unit (PC power supply): Manufacturer, product name, MANUFACTURER number