Let's find Regigigas!
A Korean player found a way how to find Regigias.
Head to the Snowslide Slope. Follow the direction of this gif. You'll find a Dynamax Den with 4 stones around. This Den has no light, none of Dynamax pokemon appears, cannot even put Wishing Piece, nothing happens. However, if you visit this place and check the den with Regirock, Regice, Registeel, Regieleki, Regidrago in your entry...
Regigigas will appear. Regigigas is Lv.100 so you need to be prepared. You can only get one of Regieleki & Regidrago you need trade with other trainers. Thanks to the korean player who found this!