but instead of "dissolve" I use "melt" seems to work better .. I do not know why

Thank you so much. I had also arrived at this solution, trying everything.
Thanks again.

Hi everyone.
I received an obj from a friend but this has a problem:
all the faces seem to be separate, so when I insert a subdivision surface I have a problem (as you can see in the attached images).
Does anyone know a solution for this?