Unable to add more subscriptions once done with sync already!

So here is the scenario,
I initialize realm with flexiSync and sync the data, That works fine. Now new requirement is to get data from a collection which is huge around 50K objects. I cannot add the whole collection to flexiSync because it will take forever for initial sync.
I tried Updating subscriptions with query with .Update and .add described in Managing subscriptions documentation. But it gets Stuck.
How I can Sync the data after once the flexiSync is done?
Using flutter SDK.

What do you mean by “it gets stuck”? How do you observe this and are there any errors on the server?