Hi Vita
Don't be sorry. We're here to help.
Once the modelling/flower paste dries, it should be hard, which is what you've got so far, which is good. When you say inserted, do you mean inserting the fence into the top of the actual cake?
If you have buttercream on the cake, then it may soften the paste due to the moisture/fats from the buttercream. I don't know whether the exterior of your cake is in buttercream or icing fondant, so not sure if it's a soft exterior or flat like my easter cake below.
Here is a 6 inch Easter Bunny Cake that I have made for the Easter celebrations. The cake is a Victoria Sponge, covered in pistachio green pre-dyed icing, surrounded by a white picket fence. The blossom flowers were pre-cut and inserted with white edible dragee balls. The ‘Happy Easter’...
www.helpmebake.com
Sometimes when you try and insert things into a cake, they break, so always have spares and always pre-make holes, so things just slide straight in.
The structure of the fence is key, as is putting it in, which is why I suggest cocktail sticks or flat lollipop sticks. That way, you build a structure that will stand up and is rooted into your cake, so it shouldn't fall over or move once in place. Dowels are the key for this, I think, as they will hold the fence upright and hold steady in the cake, and they are easy to insert.
Ideally, if it's icing/fondant, you don't want to insert the fence into the cake but rather use cocktail sticks or dowels so that the fence sits on the top of the cake and the dowels you've made insert into the cake holding it in place. To hide the sticks/dowels, you would just put another thin layer of fencing over the sticks to hide them.
Either that or cut a few slots in the top of the cake to insert the fence into it. The only risk with this is the pressure you use to push the fence into the cake. Again, the moisture from the cake may start to absorb into the paste and soften it and you want to make sure the cake is fully sealed with whatever you're inserting into it.
If you are doing grass on the top of your cake, you could roll some extra balls of green icing and slot the fence into the green balls of icing to hold it in place (similar to the way you hold a rainbow in clouds) and then pipe grass around the blobs of icing in the same colour so that the fence is only touching icing and not buttercream. The piped buttercream grass hides the blobs you've inserted the fence into, if that makes sense. See the piped grass I've done on the below cake.
This is a Meadow Cake that was a recent request from my god-daughter for her 8th birthday. Her definition was as follows:- Meadow cake:- A small scene of a meadow that includes, flowers, insects, baby bunnies and lambs with green grass. I must admit that this is one of the busiest cakes...
www.helpmebake.com
The only other thing you can do is a test piece, use a piece of the hardened fencing, insert it into whatever your cake is going to be made of/coated in and see what happens to the fence piece. That way, you'll have time see if it softens fully or not.
I'm trying to run all scenarios here because I don't know what theme you have going on.
Hope this helps.