I'm a bit confused as to the range data for MAX and 321 aircraft UA ordered. Maybe someone has more info.
I find no indication UA ordered aux tanks for 737MAX or 321NEO aircraft.

The stated range for the 321NEO is 3500NM w/o aux tanks. The 321LR has aux tanks to push it to 4000nm. XLR is 4700nm. UA didn't order the LR to my knowledge.
The standard 321neo should provide Hawaii with fantastic service, but DEN could be out of range with winds. Markets like IAH-LIM is at the edge of workable. EWR/ORD Carrebean is no problem as would be EWR-KEF, but UA will probably fly an internationally configured aircraft on any TATL route (maybe the exception of EWR-PDL.)

739ER has a range of 2870nm without aux tanks and this may be why flying it to Hawaii has weight restrictions.
Other sites show the 739ER range of 2950NM with 1 aux tank. 738 range is 2935NM.

737MAX9 has a range of 3550nm with it looks like a standard aux tank. This is comparable to the A321neo.

UA has stated the 321neo will be a domestically configured aircraft flying the same type routes as 753, 739ER, etc. This would mean Hawaii, but not TATL