You're asking a subjective question - we don't have nearly enough information to provide an answer, but the inference I would draw is that the individual is reluctant to ask for help. You don't describe the efforts you've made to communicate that help is a desired & expected transaction, but I'm going to assume that you've done everything in your power to communicate that concept.
I'd infer that this individual is one of the people whose resume includes phrases like "self-starter". Someone who believes that by figuring out the problem on their own, they'll learn and be more valuable to the company. That learning leads to deeper understanding. You probably don't want to undermine these segments of their self-image - those are valuable traits and if they can be merged with the situation, both sides will benefit.
If encouraging the to ask for help were going to be effective, it would have been effective before this. If you keep encouraging them, then you're not learning from reality, you're trying to bend reality to your will.
What are your options? What incentive structures can you build that will lower the friction of asking for help?
You're going to have to reach out past that gap. Schedule a periodic call with the individual and ask probing questions like, "What are some obstacles that I could help clear? When do you expect to finish this task and is there anything we can do to accellerate that?
Listen carefully (I believe the Chinese word is Ti fong, but explaining why that is relevant would be an essay in and of itself). You're going to have to listen past what the individual is saying and inuit what they are feeling/thinking about the costs and benefits of "help". Sometimes it will involve directly asking, "Amy is an expert on that - want me to set up a 20 minute phone call for her to walk through that with you?" - but that probably won't work; there is a significant risk that the individual will hear, "Your performance is sub-par and I have to assign another employee to intervene." - that will cause them to double down on their self-image.
Find a way to introduce "asking for help" in a win-win context. But this has to be based on deep listening.
Re-allocate the time/tasks/work package estimates for this individual to account for this behavior. If you're good, you're going to be able to shift their behavior somewhat. But even if you're excellent it is going to take some time. Do this in communication with the developer - "I'm going to allocate an extra two weeks on each of these task since you need to teach yourself the skill. Please make use of the stack and of your peers who are doing this now. Next time we talk, let's discuss whether an extra two weeks/task is the right timeframe for you to figure it out on your own. If you think that I'm overestimating the time for you to learn it, or if the resources help you to deliver faster, that would look great for both of us. But I'm only going to adjust that delivery date based on actual deliverable - it would be really disastrous for both of us if the estimated delivery date fluctuated - neither of us would have any credibility going forward."
Listen. Reward every movement in the right direction. Listen carefully to every indication that "figure it out" is triumphing over "team player".