Try using Second Screen Only? However, even when I use Second Screen Only I can tap on my Surface screen and see where I am tapping on my non-touch attached monitor. So, perhaps in addition to turning off the Surface screen you would need to disable the Surface Touch support too. A problem then would be that you would not be able to get help from a "troubleshooter" to get your other Touch screen working because all it would try to do is repair the "problems" that your intervention had caused. However, you might get lucky and find that the Dell monitor's Touch would then be working. In any case, if you can identify the Touch screen part of the monitor (probably under HID and possibly hidden) you would be able to get whatever kind of repair would be available by Update driver etc.
Oh. Touch support = USB device, so it may be under that instead of HID. In any case I would still doubt the claims seen that you don't need to have a driver.
https://www.dell.com/community/Monitors/P2418HT-Touch-not-working/td-p/6035092
(BING search for
p2418ht driver site:dell.com
)
Good luck
Robert Aldwinckle