In my job there are a lot of times where I am not doing much and slacking off. Unfortunately there is a lot of control due to security and I cannot do much on my computer (no install of software, a lot of websites are blocked by the firewall etc.).
For instance, I would like to learn a new technology, or consolidate one (for instance Node.js) that I know, but I can't because I cannot install Node.js, ore reach npm to install packages.
How can I use the time that I have available during work? What would you do?
Books are not a good idea because I don't think it would seems good to bring a book and study it at my desk
EDIT: I work in Banking and I cannot bring anything with me (such as my personal laptop). This means I am constricted to the use of my work PC. Something like repl could work, but for instance, that is not reachable for me
Your team lead might not have realised that the workload is becoming sporadic.
Speak to them, explain the situation, and ask if there are other smaller tasks you could use to fill in the gaps.
If it "wouldn't seem good to bring a book to study at my desk", then it's probably not a good idea to do effectively the same thing on the computer either.