Activo la itinerancia en el móvil de prepago.

Because activo here is a verb, not an adjective.

It is the 1st person singular present of activar: (yo) activo.

Verbs in Spanish do not change to match the gender of the object. So even though la itinerancia is feminine, the verb stays activo because the subject is yo.

Compare:

• Yo activo la itinerancia.
• Yo activo la alarma.
• Yo activo la tarjeta.

In all three, the verb is still activo.

Activa would be different:

• Activa la itinerancia = Activate roaming (informal command to tú)
• La cuenta está activa = The account is active (adjective)

Spanish often leaves out subject pronouns when they are clear from the verb form.

Here, activo already tells you the subject is yo, because the -o ending marks I.

So:

• Activo la itinerancia...
• Yo activo la itinerancia...

Both are correct, but the version without yo is more normal unless you want emphasis or contrast.

For example:

• Yo activo la itinerancia, pero tú desactivas los datos.

There, yo and tú are useful because you are contrasting people.

Because itinerancia is a feminine singular noun, so it takes the feminine singular definite article la.

• la itinerancia
• la conexión
• la tarifa

Spanish usually uses an article where English might or might not. In this sentence, la itinerancia refers to the phone feature/service as a specific thing being activated.

Both exist, but they are used a bit differently.

In Spain:

• itinerancia is the official/formal telecom term
• roaming is also very common in everyday speech and on phones/settings

So this sentence sounds a bit formal or technical, like something from customer support, a contract, or a telecom website.

Common alternatives in Spain include:

• Activo el roaming en el móvil de prepago.
• Activo la itinerancia de datos... if you mean data roaming specifically

So itinerancia is correct, but roaming may sound more everyday to many speakers.

Because en here means something like on/in the phone, referring to the device or its settings.

So activar la itinerancia en el móvil means you are enabling that function on the phone.

Why not other prepositions?

• en el móvil = on/in the phone
• para el móvil = for the phone
• del móvil = of/from the phone

So en is the natural choice when talking about enabling a feature on a device.

Because this is Spanish from Spain.

In Spain, the normal everyday word is:

• móvil

In much of Latin America, people more often say:

• celular

In Spain, you may also hear:

• teléfono móvil = full form
• móvil = the usual shorter form

So móvil is exactly what you would expect in Spanish from Spain.

De prepago is a noun phrase that describes the type of mobile service/device: prepaid or pay-as-you-go.

Spanish often uses de + noun where English uses an adjective.

Examples:

• tarjeta de crédito = credit card
• gafas de sol = sunglasses
• coche de alquiler = rental car
• móvil de prepago = prepaid mobile / pay-as-you-go phone

So prepago is not functioning like a regular adjective here in the same way English prepaid does. It is part of a de + noun structure.

Because after de, Spanish often leaves out the article when the second noun is being used to classify or describe the first noun.

So móvil de prepago means something like a mobile of the prepaid type.

This is very common in Spanish:

• tarjeta de crédito
• centro de salud
• casa de campo
• móvil de prepago

If you added an article, it would usually sound more specific or would change the structure:

• de un prepago
• del prepago

Those would not fit the intended meaning here.

Spanish often uses the definite article el/la where English prefers a possessive like my.

Here, el móvil can be understood from context as the mobile phone I’m talking about, often implicitly my phone.

So both can work:

• Activo la itinerancia en el móvil de prepago.
• Activo la itinerancia en mi móvil de prepago.

The second one is more explicit. The first one is still natural, especially if the context already makes it clear whose phone it is.

In real usage, it often refers more to the mobile line/service/SIM arrangement than to the physical handset itself.

That is because prepago is really a billing/payment type. A phone is not inherently prepaid in the same way a contract or SIM is.

So depending on context, móvil de prepago can mean:

• a phone used with a prepaid SIM
• a prepaid mobile line/account
• more loosely, a prepaid mobile phone

In everyday language, Spanish speakers often use this kind of shorthand without needing to be technically precise.

Yes, it is grammatical and understandable, especially in a telecom or support context.

That said, a few variants may sound more everyday depending on the situation:

• Activo el roaming en mi móvil de prepago.
• Activo la itinerancia en mi móvil de prepago.
• Activo la itinerancia de datos en mi móvil de prepago. if you specifically mean data roaming

So the original sentence is fine, but it has a slightly formal/technical feel because of itinerancia and because el móvil is less personal than mi móvil in isolation.