2019 Ram, new tires, manually checked and inflated tires to recommended pressure but TPMS still says 1tire is low
The tire shop where you bought tire can most likely use a scan tool to reset the TPMS . Or you can use your warranty if it is still in effect . There also may be instructions in your owners manual.
It’s also remotely possible one of the TPMS transponders is failing, although it’s a bit soon given the age of your truck. A tire shop or Ram dealer’s shop can figure it out.
Is the display showing what tire is low and have you checked the spare tire that is not shown on some dispay’s?
There is a possibility that one of the tire pressure sensors was damaged while mounting the tires.