i know words on a forum can come off as harsh, but I assure you I am not calling you any "i" anything.
its a fact that there is a learning curve when one leaves apple to android. i don't know your level of knowledge of android so I have to assume you don't know much until proven otherwise.
no disrespect intended.
there is not any "setting" per se to keep the phone from reverting back to its last known wifi state.
if you have any aggressive battery saving setup enabled on the phone, this may interfere, so if you have any such protocols to save battery or kill off background processes, revert the settings back to factory or disable any such settings.
i think if you dont use the smartthings app and just follow the directions on the product you are trying to connect to, you should have success. there's thousands of note 8 out in the wild and many of those are able to connect to smart devices just fine (myself included).
some products may not like the 5ghz band, so make sure you are on the same network.
close out any background apps--there could be some app you have installed trying to call out to the internet whilst you are doing this setup. normally you do NOT need to close any background apps on android because the system is VERY VERY good at managing system resources, but for purposes of this, close them all out.
perhaps even do a flush of your phones cache (from power off, hold power+bixby button+up vol button) and look for the cache setting. if you're not familiar with how to do this, you can find it quickly by an internet search. be careful you dont tick factory reset or anything else.
if you can't find this procedure ill link you to it.
perhaps also disable/turn off debug during these setups.
under dev options in the network section, enable all the settings in there. i have all of those enabled because I use the note in in its max state, so keeping these enabled may help during the setup. you can revert back to disabling them if that's how they normally are.