This might be late but,
It is very possible to use your phone as a Hotspot and connect your Nintendo to it to set up a makeshift WiFi spot. Its the only way my Nintendos get online acsess. Here's the problem thou
Because of firewalls and a bunch of other stuff, it will not let you host or go to MANY people's towns. Also if you go to the Tortimer islands it errors you after 5 mins on the regular 3DS. It seems the Internet is better on my XL, I don't get errored that often. You also can't update or download games using Mobile Hotspot WiFi.
The mobile Hotspot is useful for seeing friends online, dreaming (I've had very very few minor problems with dreaming after all these years) seeing what's on the Nintendo eshop, updating your dream and using the BF messaging.
Because you didn't specify how it didn't work, this is how you set up your phone as a hotpsot. Mine is a Samsung Galaxy so yours may be different if you have an iPhone but should be the same-
Go to Settings
Go to Tethering and Mobile Hotspot
Turn it on and leave it as that. Dont set up an allowed devices thing
Go on your Nintendo to the Internet settings
Search for your hotpsot name
(Your Hotspot should be locked to prevent other people using it) Enter the password your phone provides you for your hotspot.
Wait for it to do its connection test and BAM you have makeshift WiFi. Now you can do the above mentioned.
Feel free to message me or ask me any questions about mobile hotspots~
Four corners Four towns
Moriyo Village, Treehut, Secret & Luminosa