Samsung Galaxy S10+ Will not boot into Recovery mode from Download Mode
The problem with the S10 is that you cannot shut it down from &quot;out of the Android OS&quot;.
Then you can only boot into the recovery with the (hard-ass) key combination when your phone is completely shut down. So if you miss the tricky key combination one time after shutdown or odin-flash you're stuck in some kind of boot loop between the &quot;bootloader&quot; and the &quot;security warning&quot;. The only solution seems to be to let the battery drain so the phone is completely off and then use of one of the following methods:

Booting Into Different Modes
Even though you have rooted your Samsung Galaxy S10, you still have to
keep in mind that both the recovery and the Magisk will be in the same
partition. Which means that you will have to use a different method to
root into the rooted system.
Note: The following steps are to be followed when your phone is
completely powered off.
Unrooted System: Simply hold the power button as you would normally and your phone will boot into an unrooted system.
Rooted System: Hold the Power, Bixby, and Volume Up buttons together, and release them once the Bootloader Warning screen appears
and that will boot your phone into the rooted system.
Stock Recovery: In order to do that, you will have to hold the Power, Bixby, and Volume Up buttons together, and release the Power
and Bixby buttons when the Bootloader Warning screen appears and keep
holding the Volume Up button to boot your device into the stock
recovery.

In your case it will be the last option which is the most tricky key combination ever. I also failed it several times, it needs practice! (I suppose Samsung did this on purpose).
Check this guide, it's the best I found out there:
Root Samsung S10 Guide