Theoretically, maybe you can set it to auto, but in certain circumstances of bad reception (frequently indeed), putting it manually to 3G-only gives better results according to my experience. Why, I don't know, but that works better. In good (normal)...
One thing I forgot. You say your telephone works when youhave a signal. But it may be also that it is set to accept 2G as a minimum. To be sure, please set your phone to 3G as a minimum (3G/LTE). 3G-only as a setting may also help as LTE's signal is ...
Was it set to 2G, 3G and LTE before, and is it set the same now ?Modes change the range of transmission and therefore can create bad reception when another mode would be OK. Try 3G, which is the minimum (at least where I live).
Je crois qu'il suffit de taper * très vite après avoir entré le mot de passe. J'ai eu le même problème au début. Le menu vocal est à mon avis déficient d'un point de vue interface de l'utilisateur.
