Connecting GS728XP to Dell N Series Switch using Fiber not working

There is no such beast like a GS728XP. GC728XP probably? GS728TP? GS728TX?? GS728TX?

When I have it right, the N3048P does have 2x SFP+ ports (front) and some SFP ports (back).

Using SFP or SFP+ modules - and if yes, in which ports?

SFP+ can only be connected to SFP+ when using 10G fiber modules.

SFP can only be connected to SFP when using 1G fiber modules.

under some conditions, SFP+ caches can accommodate SFP modules so they can be connected to a correct SFP/1G or compatible SFP+ port with SFP 1 G module be connected to SFP when using 1G fiber modules 

Mixing different ports (caches) with random SFP modules does not work at all, no fiber link comes up.

Some switches as like the Dell are picky with the module brand and model name in the flash, non-supported lead to port disabling. There might be ways to disable this "lock".

Some switches have SFP and/or SFP+ ports (caches) predefined as stacking, not as general Ethernet ports - so again no fiber link.

Dirty fiber connection ferules can lead to cumbersome or no fiber link.

fiber modeless must support the same technology (single mode or multimode), must have the same technology on both ends, and of course TX and RX the same colors as the counterpart RX and TX.

Welcome to the wonderful world of fiber basics.