^ same problem with Warid.. 4g coverage in big cities and central areas only.. so if your traveling is karachi ISB Pindi lahre etc. warid device and zong are good to go.. otherwise mobilink and telenor have the biggest coverage map overall..you can search their 3g maps on their websites, so you would know which company suits your need
as for your actual question, about cities, the proper answer is.. YES< you can use one device bought in balochistan and make it work in islambabd and upto kashmir.. as long as it is in the signal coverage area. no issues..all networks are standard all over pakistan.. package changes may apply in specific cities, as warid has different promotion for lTE and ufone for its 3g in different cities, like ISB and Pindi have a different cheaper package than lahore, so if you have a bulk package nothing to worry about.. daily and weekly bundles may incur different charges.
the unlocked device system means that you can buy an unlocked or hacked device, which will then work as a carrier free 4g router ..so you can put any sim from any other network and get it to work as well, so warid telenor and mobilink sims could be used interchangeably in same device... or you could simply buy a simple 4g router from any shop, which is already carrier free and works anywhere worldwide.