I am not able to change de contact number on the support app

If you're not in the US, you must change your country and region settings first in order to use a phone number from a different country

See this on how to do that

1. Open the Settings app. 
2. Tap your name, then tap Media & Purchases.
3. Tap View Account. You might be asked to sign in.
4. Tap Country/Region.
5. Select your new country or region.
6. Tap Change Country or Region.
7. Tap your new country or region, then review the Terms & Conditions.
8. Tap Agree in the upper-right corner, then tap Agree again to confirm.
9. Select a payment method and enter your new payment information and billing address, then tap Next. You must enter a valid payment method for your new country or region.*

Please be aware that you need to fulfill certain requirements in order to change your country or region '

• Spend any balance remaining on your Apple ID. If you're not sure whether you have a balance, check your Apple Account balance.
• Cancel any subscriptions that block a country or region change, and wait until the end of the subscription period. 
• Wait for any memberships, pre-orders, movie rentals, or Season Passes to complete, and wait for any pending store credit refunds to process. You can cancel a pre-order.
• Make sure that you have a payment method for your new country or region. See what payment methods you can use.
• Redownload your apps, music, movies, TV shows, and books on any devices that you might use to play or read them in the future. Some types of content might not be available in your new country or region. Learn what types of content and services are available in your new country or region.
• After you update your Apple ID billing address and payment method, you can upgrade your iCloud+ plan using the pricing in your new country or region.