Hi and welcome.
Yes, I think you are expecting too much too early without enough focused training (very easy to do -- we almost all make this mistake!). Recall generally takes many weeks/months of actual training (no dog just instantly knows we want them to come back! It is a long and cautious training process
) and a dog should not be allowed off lead until you have this carefully and safely trained, starting on a long lead in the garden and working up to places with more distraction such as a park.
It goes without saying that no dog should ever be offlead on walks on city streets or rural roads as no dog has ever been trained that could be considered 100% reliable --there is always something somewhere that could distract a dog and no one wants to risk a death!
For a new rescue dog the risk is especially high that he will head off and never return -- especially as he has only had a week to start to learn a new name, which is also a process that takes time. He needs many weeks generally to really settle in. From years doing rescue, I know of too many tragic cases of rescue dogs bolting because they just haven't fully settled in with a family, don't really reliable know to answer to their name, and don't have much if any recall.
If you look in the training section there are many pinned items at the top that list great training pages which all have info and often videos on training recall. You can also download a fantastic free book that is great for puppies, rescue dogs starting out on training, or ANY dog of any age, called, After You Get Your Puppy:
www.dogstardaily.com/free-downlaods
Many petshops and online sites carry long leads -- which are 25-50 feet leads for training dogs. Extensa leads (the ones that recoil) are NOT appropriate and can even be dangerous as they very commonly cause a dog to bolt if accidentally dropped (the holder easily scares dogs when they find it bouncing along behind them).
Enrolling in a good rewards-based (not punishment/correction based!) group obedience class will also really hasten your bonding with your dog and give you great skills for working with him and he will have fun too. Look for a CPDT or APDT certified trainer.
Once you have a dog that has safe recall you will be able to enjoy so much more together!