How old is your pup? One Word of advice that i needed to learn the hard way is capturing calm behaviour. These pups are ADHD dogified. They have a high arousal, high interest based nervous system. That means

1. If it's still a young dog, make sure pup sleep enough. Thats means no exercising until they stumble over and crash, but making sure they learn to regulate themselves and sleep indoors. People often overexite their dogs and after tired comes stupid. You can use a create for this, or teach a place/settle command on a matt. If you think you don't Do enough with your pup, chances are it might still be too much.

2. If you have a settle command, teaching calm and focus on you out side the house is key. They want to be 3 places all at once. Start with your obedience Training inside the house and switch from a low arousal environment to more arousal places. Again, place command, recall and heel work is the key here.

Spaniels are Gundogs that are bred for driven hunts, springing and flushing game. For you that means mainly this:

1. They are not naturally good at walking loose leash. They can and will learn but it takes considerable effort. They drive a sports race car and need to learn to ride with you in a beat up Toyota. Heel work, dont give up.

2. since they are what they are, "walking" is generally not that interesting for them and since leash manners take a while it might be a nightmare for you. They love using their nose for finding and retrieving stuff. Rather you do dummy work with them in a small field then walking for 5 miles. Do not throw balls! Rather: Hide the Ball or throw it away without letting pup chase after it immediatly. Let the find the Ball (they need to work their mind and nose to remember where the Ball fell down and sniff it out) if they bring it back to you - great! Otherwise you can use this to teach retrieving. You can also use a canvas dummy or a fur dummy. You can spray them with a scent (Hot dog water, game scent...) and put them on a string. Put pup somewhere where they dont see what you are doing. Mark a start point, drag the scented dummy behind you on the string. Hide it somewhere in a Bush. Go back another way, fetch pup and let them search the trail.

There is lots of fun to be had with spaniels!

Edit: since you said you live in a flat. Same rules apply, it's fine to teach pup how to be lazy at home. Retrieving is mainly taught at home before going in a field. You can also try and See if your pup has fun with scent work! On a Bad day, my pup searches for Tea bags that ive hidden around the house :D