Iam having the same issue with an amazon fire 10.1 tablet, if anyone can help would be a blessing ,
Using new smartthings hub not old one , so i cant try to log into the classic one bexouse the new hub doesnt have a code , so iam stuck trying to ise the new app
I had same issue on Mate 20 Pro - basically there wasn’t a ‘default’ browser installed, I had chrome, chrome beta and FUll browser, after I deleted Beta and Fully all worked fine
On the amazon fire tablet 10 inch 7th gen that iam using i added a default web browser , fire fox but silk browser still keeps taking over and sending me to that page , i can open up the classic app its asking for an activation code and the new hub doesnt come with one 
I resolved the same problem on my Honor 7x. I uninstalled “duckduckgo” browser and left Chrome as the only default browser option. Bingo. I was able to login.
Anyone know if these steps will work for Fire tablets?
Same problem for me on a brand new ASUS Google Chromebook.