Yes, you can add/remove and activate journeys without internet however the app requires a connection every 2 days.

But you need an internet connect every time when you activate a new travel day if you have a flexi pass and if you have a continuous pass you need an internet connection to activate your outbound/inbound travel days. And you need an internet connection at least every 72 hours.

Okay, I have a 10-day in 2-month flex pass.  I understand that I need an internet connection every 3 days, no problem since I use the internet every day. The directions tell me If I try to add a journey to my pass and I have not activated a travel day the app will ask me to activate a travel day. The morning of the day I want to travel I activate a travel day while I have internet access at my hotel but do not add any journeys to the pass at that time. I have added 3 possible train journeys for that day to my Trip but not to the Pass. One of my selected train connections no longer appears in the schedule on Eurail so I wait until I arrive at the station to check the Board to see if that train is available before adding a journey to my Pass. Now I am ready to add a journey to my Pass from the list I have saved in my Trip but there is no internet connection at the station and i do not have data to connect to the internet. Will I be able to add a journey from the list in my Trip to my Pass even though there is no internet connection.  One of the previous answers says yes but another one says no. Which one is correct.

After you've activated a travel day AND generated a QR code by adding a train, you don't need internet anymore. You can add or remove trains as you wish.