RDS stuck in modifying status for 20 hours

It looks like you are converting this instance from io1 volume type to magnetic.
You recently requested a point in time recovery, but you've specified 'standard' volume type with is magnetic storage.
In order to complete this conversion, it requires an expensive io operation. This is why your instance is in modifying, it is completing the conversion.

I'd recommend you consider gp2 as a better performing option rather than magnetic.

-Phil

As per your advice I changed from magnetic to GP2 and it's been in modifying status for more than 60 hours.

Most scale storage operations take effect immediately, however b/c you have a legacy volume geometry, it will require a long running io operation.
Once this completes you will have the new volume geometry and any future scale storage will execute quickly.
There is no problem with the current instance, it is just completing the conversion from magnetic to gp2.

-Phil