The OHKO moves (such as Horn Drill) won't have any affect on a Dynamax Pokemon,
except when they have a shield.
The move removes 2 shield points (rather than the usual 1),
but it does no damage if they do not have a shield.
The Weight Moves (such as Heavy Slam and the Grass Knot you mentioned) will also not have any affect, your Pokemon will not use those moves against Dynamax Pokemon as Dynamax have a weight of ??? as shown in the Pokedex.
It seems that moves that affect the held item, such as Knock Off and Bug Bite don't work as well.
Also moves that affect the Pokemon's ability, such as Skill Swap will not work.
Lastly moves that send the pokemon away, such as Whirlwind do not work.
Paul's answer is certainly great with that list, however Serebii actually has a second list on a different page that includes the weight moves and others.
As mentioned in Pyrite's comment, Curse (when used by a Ghost type) does actually work.
Here is proof I recorded.
So in total it seems the list of moves are as follows:
- Bug Bite
- Circle Throw
- Destiny Bond
- Grass Knot
- Heat Crash
- Heavy Slam
- Horn Drill
- Knock Off
- Low Kick
- Perish Song
- Sheer Cold
- Skill Swap
- Super Fang