Since we need to compare the tree T with every subtree of S, we can do deep first search to find every subtree of S.
For each subtree we compare whether it has the same structure as T. We can use a preorder transversal for this.
The running complexity is O(n) to find n subtrees in S and, for each subtree, O(m) to compare both trees, therefore, O(n * m).
The space complexity is O(h + m), where h is the height of S (since we are using recursion, which go down h times) and, for every subtree of S we create 2 stacks os size m to compare both trees.