Zannfour is a totally weird place. It contains the huge Roman city of Assuras, where nothing has been excavated. A few monumental structures peek through the fields where sheep graze. All around lies stones and remains of pillars, many with fine inscriptions.
Nothing of hotels or restaurants. Your best choice is Le Kef 55 km northwest. People who can tolerate any hotel standard, can spend the night in Maktar 35 km southeast.
You will be able to get to Vieux Sers by shared taxi from Maktar or from Le Kef. After that you have a problem, as there are few vehicles going out to Zannfour. The best will probably to rent a taxi for the return trip. Then you can also throw in Elles, 10 km away, which has some very interesting megalithic tombs.
Should you drive your own vehicle, follow the unsignposted road forking out to the west of the main road at Vieux Sers. After a few kilometres you'll reach junction, take the road to the right and follow this until you get to the small canyon where Zannfour lies.