Topic: How to localize the reason in a network 4G?

Bought in France sim card La Poste Mobile (the virtual operator on the basis of SFR). In mine Xiaomi Redmi Note 3 Pro (Qualcomm) works only 3G. At switching in 4G/LTE in the status it is permanent hangs "No service". In  and the 4G-modem this sim card normally is registered in networks LTE, and other sim cards LPM in my phone in LTE too are not registered. In a mode of manual registration all LTE-networks are visible, but registration in network La Poste does not transit. In arguings problems with APN are often mentioned, advise manually to change APN on "sl2sfr" and to disconnect IPv6. After long shamanism it, at last, helped. From here a question: How in  (and in iOS too) to distinguish failure at setting of mobile connection (mobile data) from failure at registration in a mobile network? When the mode is forcedly restricted only 3G, it is visible at once - the network tag is, the information on it is. If to force down APN - mobile connection disappears only. And in 4G - one lines "no service". Applications like Cell Signal Monitor too show nothing.