My Doro 508 says invalid sim what can i do to fix it
That may be so but the sim is the issue not the phone, did the sim come with the phone. If it did does it require activation with the service provider in order to activate it's services. If it is an old sim, i.e. one that has not been used for a considerable time, it is possible that the number is now obsolete or has been reassigned.
It might be that the sim is incompatible with the phone, is the handset compatible with the sim, i.e. if it is locked are they for the same network.
Also, the sim might be damaged, attempt to use it in another, unlocked, phone in order to test whether this is so