2018 Nissan Altima radiator fan never comes on

Yes, the fan should come on with the AC running. On your 2018 Altima the cooling fan is commanded on whenever the AC compressor is engaged, because it has to pull air through the condenser. If the fan never runs with the AC on, something in the fan circuit is wrong.

The temp gauge staying normal is actually a useful clue. It means the engine is not overheating, so the problem is most likely electrical rather than a cooling system failure. Work through the circuit in order.

Start with the cooling fan fuse in the under-hood fuse box. If it is blown, replace it, but know that a blown fan fuse usually means something drew too much current, so watch whether it blows again. Next, find the cooling fan relay and swap it with an identical relay from the same box, like the horn relay. Turn the AC on and see if the fan wakes up. That swap test takes a minute and rules out the relay for free.

If the swap does nothing, unplug the fan connector and check for voltage there with the engine running and the AC on. Power at the connector with a fan that does not spin means the fan motor is dead. No power at the connector points back to wiring or the relay circuit. Also check that the coolant is full, since low coolant can keep the temp sensor from reading correctly.

One safety note: keep your hands clear of the fan area while testing, since the fan can start without warning.