R&D tasks have a higher degree of uncertainty and are generally not a good fit for a WIP limited Kanban board. It can end up creating a bottleneck and blocking other work.
R&D tasks can often be handled as part of designing and planning specific scope. For example, there's a big, thorny problem. Spend time designing and working through it. Then, once there's a handle on it (a higher degree of certainty) start chunking it into smaller stories. Those can be threaded through a Kanban board. If they get stuck in a column (push up against a WIP limit) it can suggest its not well enough understood or small enough a story.
There are R&D tasks which are too big for the above mentioned approach. Those with a really high amount of uncertainty. Those are best done in a separate, non-Agile, workstream. Do track and timebox the effort as well as expectations of where the effort should be at a certain time. For example, after two weeks we expect to have a design document. Then check in to see where that milestone has been met. Whether it has or hasn't provides information on the level of uncertainty around the task and can help decide next steps.
Were the team to have a relatively large number of high uncertainty tasks, compared to work that can reasonably flow through a Kanban board, it may be time to recheck alignment between stakeholder expectations on scope and the team's capabilities.