Seat reservation Portugal

Hi, I am from Germany and I am planning my trip end of May/beginning of June in Spain and Portugal.

I wanted to buy the seat reservations for two trains in Portugal, also because they are cheaper than I thought with just 5 Euro.

But in the Railplanner App, it tells me as the only option I have to buy it is in person in a train station.

I went to a DB-Service Center here in Germany and they told me they officially cannot book seat reservations for Interrail in Portugal anymore and they told me I need to buy them in the App, but as I said that isn’t possible.

I looked up that for my route from Porto to Lisbon, I can take 3 trains without a reservation at all.

But between Lisbon and Faro I cannot choose a connection without seat reservations, it’s the only possibility according to the app.

I‘m a little anxious now because I already booked the hostels, next thing I wanted to try is to book the seats via a Portuguese Website but I was hoping someone here with similar experience could help me out.

thanks in advance 

Luca