I believe the best option would be to look at real life 'fish out of the water'. In real life, there are two main reasons for being such a fish, IMO.
1. Lack of interest
I'm not particularly old, but I sometimes feel completely out of my depth while talking to my students about some topics. For some of those topics, I'll make an effort and learn about them, but for others I simply don't bother. When my students talk about these or those youtubers, I have a general idea who/what they are, but the details mean nothing to me. Hip-hop or reality TV shows are two other points I'm not interested in, and so I have no idea who does what where or when, and why it is considered funny (sorry, guys, but watching a youtuber teach you how to open a door is not hilarious in my world).
My lack of interest means that I cannot fully participate in some conversations, even though my students get a kick out of trying to explain what they're talking about and why it's funny.
This could relate to a character that is dropped in a new society and decides, for example, that all they need to know is that there are two political parties who hate each other's guts. When people around discuss politics, they'll expect the character to understand what is going on. In fact, the character can even make strange (or plain dumb) statements based off the too little knowledge possessed.
If possible, try to detect friends who are 'fish out of water' on a particular topic and see how they act about it.
2. Learning difficulties
This time look at the elderly you know. Most will have great difficulty keeping up with technological evolution, but there are quite a few that at least try to get how the basic works and some even become quite profficient.
Or you can look at the younger generations. If you work with children or teenagers, you'll know that some will learn some topics quite easily, while others have great difficulty. I don't mean they're dumb at all, mind you! For example, I was a very good student at most subjects... except electricity. Even today I cannot grasp some stuff which everyone around me considers absolute basics. That means, I am a complete 'fish out of the water' every time someone decides to talk about electrical wires and people will sometimes look at me like I'm an alien.
Do note, however, that what is obvious to you is not obvious to whom you're teaching. Just try and let an 'expert' on a topic teach you something you know nothing about. Either the expert is a born teacher, or they'll quickly lose patience with how slow you are at picking something so basic. The problem, though, is that one needs time to...
understand the logic,
memorise all the steps (I'm thinking about how to send a text),
consolidate the process so one will remember it next time (it's easy to get how to do something, only to find out you've forgotten where to go and what to do next time you need to do it again)
Unfortunately, most 'experts' teach like this: you go here, then you click here... and here... and here. And that's it!
When writing a character out of their depth, the key thing is to make sure they act like normal people, so look at people around you and see how they act when they're out of their depth.
The problem is that most people are 'fish out of water' occasionally, while your character will be constantly on a tight spot. Show the effort of the character as they try to soak it all up, show their confusion when the loads of new information start getting mingled into a huge mess, show their frustration with the people who teach too fast so the character never gets a chance of understanding how things work... or simply say how it's done but never give the character a chance to actually do it so they can never fully consolidate the new information.