Some original game monsters (and an injoke around the 80s cartoon series) will appeal to old fans, but a limp plot, predictable characters and a lifeless script keep this film from the real magic.
D&D has never convinced on the big screen because it misses out the things that make the game appealing: the dice rolling, miniatures and maps; the feeling of being the star of an improvised story; and the contradictions between character and player interaction.
The latter is especially difficult to conjure up, but is where any other movies in the franchise need to look for inspiration. Imagine the D&D scenes from ET or Stranger Things and work towards something like a heroic fantasy Galaxy Quest.