There is no direct bus to the Great Wall from Tianjin and here is my recommendations: take taxi from Tianjin Port to Tanggu Train Station and then take bullet train to Beijing South Railway Station. In fact, most bullet trains depart from Tianjin Railway Station and few from Tanggu. Notice the train time. You can find at this page:https://www.travelchinaguide.com/china-trains/
Then take subway Line 4 from Beijing Railway Station to Xizhimen. Get off from Exit A and you will see Xizhimen Railway Station (also Beijing North Railway Station). There are bullet trains (Y trains) to Yanqing every 1-2 hours. You will get to Badaling Great Wall one hour later.