I give u an example about wifi with 不管用： u want to use the wifi to connect the internet. what u use: the wifi the goal: to connect the internet result: u COULD connect the wifi but u CANT connect the internet. Now u can say: 这个wifi不管用 But for most cases, it's that we couldn't connect wifi. we dont highlight what we use, we just want to connect the wifi. so we say: 我连不上这个wifi. We want to express that I got a problem when I connect the wifi. We dont highlight what we are using and the goal of connection. So we say 我连不上wifi. instead of "wifi不管用"

another example with wifi: - could u give me the password of ur wifi? - OK, here it is. - No, the password is wrong / the password doesn't work. --- Here, u can say: 这个密码不管用. u want to use the key to open the door, u want to use the password to connect wifi, but u fail. in this case, we use 不管用. It highlights that what we use doesn't work. BUT for "wifi doesn't work", we use our phone or our pc to connect wifi. our phone or pc works, the router(wifi) works. So we cant use 不管用 in this case.

isn't working=不管用

doesn't work in chinese is 坏了. for example, the car doesn't work: 这辆车坏了. the phone doesn't work: 这个电话坏了. when something doesn't work and we dont know why, we always say: something 坏了. for the wifi, we rarely say wifi坏了. we always say: wifi连不上了. or 我连不上wifi了. why do we rarely say "wifi坏了"： because for most cases that we cant connect wifi, the reason is always something like: password error, configuration error etc. In other words, wifi works, ur PC or ur phone works, the router works, everything works we cant connect wifi because we dont make the configuration right. so u see, it's not "坏了". 坏了 means "be broken", if we cant connect wifi, nothing is broken, it's just because of some configuration. We say 汽车坏了, maybe its tyre is broken, or its engine is burnt etc. U see, there are cases "坏了". So if a car cant run, we normally say: 汽车坏了. BUT for the problem of connection of wifi, the hardware is rarely broken. so we dont say "wifi坏了", we say: "我连不上wifi了" or “wifi连不上了” ---- it means I cant connect the wifi. Lynn said: 不管用. this is for another case. Maybe in Hong Kong they say "wifi不管用", I come from the mainland of china and I ve NEVER heard this before. We DONT say like this. 不管用 is used for this case: u want to use something to achieve some goal but u fail --- nothing is broken. for example, u can open ur door with ur key. Now the lock is changed by ur mom, and ur old key doesn't work anymore. In this case, we can say: 这把钥匙不管用了。----this key doesn't work. U see, the lock works, the key works, they are not broken at all.