Iphone xs max ios 14 secondary card has no service. how to solve it?

First of all, see if you bought the unlocked version. If it is an American contract machine, it can't be used at home. Secondly, this is a common problem of iphone. . You take out the card and insert it again. You can either turn on the flight mode for a while or turn on the machine again. It usually takes a while. I really can't call Apple and have it repaired for free within one year. It is possible for the United States to directly replace the new machine.