Dell Latitude 5420 to Dell 5477 monitor setting not allowing full resolution

I have a Dell 5477 that I use as a second monitor for work and with my Latitude 7480 I can use the full resolution on the monitor portion of the AIO 1920X1080.

I just got a Latitude 5420 and it will not let me go higher than 1360x768 when using the Dell 5477 as a second/extended monitor off the 5420.

After some work my support guys advise the video on the 5420 is only 1gb where the 7480 was 2gb and this allowed my 5477 monitor to give me full resolution.

Is there anything that can be changed on the 5477 to allow the laptop to allow full resolution at 1920x1080.

It works fine on two dell monitors and an Insignia TV.  What about the 5477 requires more ram for full video resolution?