do need a different cell plan to connect to my Apple Watch 9?

There are 3 modes of Apple Watch operation

• Cellular model paired to your iPhone, and shares your iPhones number. If your iPhone carrier supports the Apple Watch, then this generally adds $10 U.S. to your iPhone bill.

• GPS only mode, which can be a GPS only Apple Watch, or a cellular model that you decided not to activate the cellular. In this case the Apple Watch just uses the iPhones radios to make the phone calls, as long as the iPhone is nearby, which is typically somewhere in the same moderate sized wood frame construction home. This mode can be used with any cellular operator, including NVMO operators that do not official support the Apple Watch, because the Apple Watch in this mode is not attempting to use cellular in the Apple Watch.

• Family Sharing, is a cellular Apple Watch given to a dependent child (or an aging relative), and the parent's iPhone controls everything on the Apple Watch. Because the Apple Watch does not have its own iPhone, it loose many features that are dependent on having an iPhone of your own, and the cellular plan from a carrier that supports the Family Plan is much more expensive.

ChickR wrote:

So it’s not the gps only model nor the model that has its own number. I’m talking about the series 9 which shares its number with your iPhone. ....

The series 9 aluminium come in two types

The Stainless steel model does not appear to be available in GPS only,

The cellular model does not share its number with the phone if you activate the cellular.

Bob explained it better.

If you buy the cellular model which will have its own number, yes EE will have to add the watch to your phone plan for a monthly fee (£7 pm??)

MVNOs like GiffGaff or Lebara cannot provide this, only EE, O2. Voda, Three.

The gps-only model can take and make calls if your phone is in bluetooth range. Not many people actually go out with just their Watch, maybe joggers. In my opinion not wirth tte extra money.

So GPS-only model, no you can keep your MVNO provider on your iphone.