dell precision 7710 dual monitor mouse not working across monitors

That's weird. Do you use enough "force" to jump the invisible dividing line between the monitors. You have to "bump" over this line, because Windows built this in, just in case you _DON'T_ need to cross this line by accident.
So in essence, just more force, in most cases, speed up a bit with the mouse/touchpad motion, when crossing to the second monitor.   

Doesn't make sense to me.  I would question if the monitor is switching places and the computer thinks it's on the left and not right.  Or perhaps top or bottom.  Would love to see/hear an illustrated little video of the problem to help envision what's going on because there's no way this should be happening as I'm currently understanding, other than the monitor's "location" is lost.

Above is easy to test, when having the display properties and using the Identify button (big 1 and 2 are displayed on your screens, besides the fact that the screens already had their brand/model name displayed in the properties)

Taking a run up seems to work

as you can see from attached pic the Right screen is Bigger than the right could this be part of the problem if so  where are  the settings to adjust?

Right click on an empty part of the desktop.and go to display settings.
Arrange where you want your monitors configured (click and drag on them to re-arrange their location).
The size differences (assuming the monitors are otherwise identical) are probably due to the Scale and Layout settings (scroll down on the display settings).  Whatever monitor is "blue" (as in my image above) is the monitor you're adjusting settings for.  Click each and make sure the settings match or are set to whatever you want them to be for that specific monitor.

@trevor1940 Your screenshot seems to indicate that either the screen resolutions are different (meaning, you can NEVER align it properly, since it's simple math) or they are the same resolution, but you'll see the screen alignment is slightly of in the display properties (one is slightly higher)
If the screen resolution of the right screen _IS_ lower, basically, where they align in the display properties, is the only place your mouse can "pass". If you align the right screen to the upper right corner of the left screen, you can pass for left to right, only at the upper part. Say the left screen is 1920x1080, and the right screen is 1440x960, it mean, for the right screen, there are about 120 pixels at the bottom where you CANNOT pass to the right (if you align it to the top). If you align it to the bottom, it's the other way around, there's a part of the top you cannot pass to the right. If the right screen is 1280x720, there will be an even bigger part where you cannot pass.
Or if you take the screenshot above from @Lee, in screen 2, the upper part, using your mouse to the left, will not work (only in the lower part of screen 2)

It's a very blury picture, but if you note, the icons in the taskbar are smaller on the right than on the left while the command prompt window is larger. That suggests different settings such as the monitor on the left may be set to 100% size of text, apps, and other items while the one on the right might be set to 150% size of text, apps, and other items.

See Attached

I've now set the left to match the right that  seems to be better

I'm going to have to play with these settings

Yes it's clear the lower part of screen 1|2 will NOT pass to screen 3. You'll have to automatically keep in your mind to move your mouse in the upper part of the screen 1|2

The Maximum Display Resolution of Laptop and monitors is 1920x1080 pixels so unsure how or why the recommended of 1/2 is 3840 x 2160 @ 250% ?

I'm not able to set monitor 3 to this so assuming my math is correct and it works that way I've set both to 1920x1080 @ 125%

Changing the percentage is ONLY for personal preference. What I explained you with the edges and where the mouse can "pass" will ALWAYS apply no matter your viewing preference.
You ALWAYS set to the recommended screen resolution (if detected correctly, and it's the specified NATIVE resolution of the monitor). You only change the resolution to lower (you can change to higher, basic science) if you want to waste your money (setting it lower, might as well have bought a cheaper monitor), and also have a FUZZY screen.

Not totally sure the recommended were detected correctly as I said The Maximum Display Resolution of Laptop screen and monitors is 1920x1080 pixels

not 3840 x 2160

Thanx Stay Safe