Infiltration is made more difficult by the fact that every
member of the cell will know each other and trust each other
explicitly, and
no outsiders are allowed, no matter how enthusiastic they may seem.
Most cells are completely anonymous to each other, and often several may be operating in the same city at the same time, and not knowing of each other even though the members could share a place of work. Cells are easy to organize, and very little is written down.
Equipment is stored in dumps usually buried dumps a decent distance from a meeting place. The meeting place for a cell is usually non-existent in the conventional sense, and it is likely that they never meet all together, or in the same place twice. Anonymity is Key. Most will have false documents, fall back identities, including passports for easy escape should the situation arise.
Everything is usually flammable or booby trapped so that intuders don't get access to the information. Life isn't easy for cell members, they will usually have been training as recruits for years before being allowed to join a cell, and after some guided missions in which their abilities are assessed and noted. Only the most commited and talented are sent to cities where cells are in any sense active. They usually make a commitment to die for the cause rather than betray anyone, and this comittment has to be real. Cells are incredibly hard to find, almost impossible to track once found, independent of each other, and usually composed of highly intelligent and extremely resourceful individuals (often women as well as men) who possess an encyclopedic knowledge of their territory.
Cells may often work and plan in one city only to move halfway across the world to take action and then disperse.
The whole thing is very fluid. Making conventional war and intelligence gathering almost useless.