ok here comes the summary
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As I said I even tried to turn off wifi on mu iPhone to get off the DIR-626L with no luck
The day after at a café in in town I was waiting for a friend and tried it the Sharepoint Mobile app again
without having changed a thing, not in the router and not in the app
and it worked ... sigh...
and I have no idea why
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since then it works, tried cold booting the router and still works
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so I have no idea what was going on :/