As usual: where it is unspecified discuss with the DM
It's worth looking at the Sphere's description to resolve this question. Since it does not contain that explicit provision - how far it can be from you once you take control - you'll need to get a DM ruling and/or (within the context of the game world) engage in a bit of testing. FAFO.
The sphere is stationary until someone controls it. If you are within 60 feet of an uncontrolled sphere, you can use an action to make a DC 25 Intelligence (Arcana) check. On a success, the sphere levitates in one direction of your choice, up to a number of feet equal to 5 × your Intelligence modifier (minimum 5 feet).
On a failure, the sphere moves 10 feet toward you. A creature whose space the sphere enters must succeed on a DC 13 Dexterity saving throw or be touched by it, taking 4d10 force damage.
If you attempt to control a sphere that is under another creature's control, you make an Intelligence (Arcana) check contested by the other creature's Intelligence (Arcana) check. The winner of the contest gains control of the sphere and can levitate it as normal.
The proximity requirement for gaining control is absolute, but once control is gained there is no further detail. The talisman just gives you more range/distance for the sphere movement. There isn't even a constraint of "that you can see" in terms of sphere control, but given how it moves you'd likely need to see your target, or know its location, to apply the sphere's damage on it by moving the sphere there.
The sphere levitates, but you aren't using the levitate spell
If we look at the spell levitate to inform sphere control, you would need to see anything to levitate it...which could include the sphere but the sphere's description does not directly point to the levitate spell.
One creature or loose object of your choice that you can see within
range rises vertically, up to 20 feet, and remains suspended there for
What is for sure is that the sphere's movement is constrained by the controller's being equipped with the talisman, or not.
Basic movement, or Talisman enhanced movement, it has only one of two rates of speed/movement.
How far are you willing to move it before you begin to worry about someone else grabbing control form you? That's a player choice, in terms of risk/reward. And first, you have to find one that is uncontrolled, or, take control from someone who has one under control.
That involves risk.
The particular point of "radius of control" appears to be left open ended and vague deliberately.
FAFO. What is your willingness to take risks?
Less risky answer for DM's pondering this
Constrain it to 60' for a control radius. I've only seen one of these in action in a 5e game, and the looming danger it presents to pretty much everyone during the encounter was a crucial feature of its presence. In that session, control was only possible within 60', per the DM's ruling.
Having it wander "off screen" to destroy beings takes away some of the tension and sense of danger.