WIFI problem only on 2.4 GHZ

Customer of mine called me today with a WIFI problem.

This is the story:

At home they had a problem with there internet providers modem that broke down.

A day later the modem was replaced and everybody had WIFI again.

But not my customer her iphone was not able to connect to the WIFI.

tonight they came in with the iphone and i tested it and this is what i found,

5 Ghz WIFI works without any problems.

2.4 Ghz WIFI works only after a hard reset and the recieving level is one bar and that should be 3 bars because the router is next to my workspace.

I hope somebody can give me a good explaintion of what the is the cause of this problem,

I do not believe in just swapping parts so i hpe somebody has a idear,

With regards

Francois

Netherlands