testing USB tethering on my dual-sim Mi5s phone, it seems that I can not get it to work with the Swisscom SIM card here, whereas all works fine with the second SIM.
Connection to the internet on the phone itself, e.g. web browsing works fine.
Inserting the Swisscom SIM in my older Samsung Galaxy Alpha, the tethering works fine.
When comparing the APN settings on the two phones there are some tiny differences (proxy/port set/empty, authentification empty) , but I could not succeed to make it work. On the other hand: once the phone is able to access the internet, the APN should be ok, as far as I understand.
So currently I do not have an idea where to search and would be grateful for pointing me in the good direction.