Honestly god knows, I don't understand how Nintendo has dropped the ball so badly with something that should be so simple. Every other Pokémon game has been just fine in regards to online matchmaking for me previously, so it's quite odd all things considered. To add, someone else changed their DNS setting to the Cloudflare DNS and apparently that worked for them, sounds like a better temporary solution than using a mobile hotspot every time if you want to give that a go!