To OP:
You already have a dual band router or you would not have both 2.4G an 5G bands. I am surprised you could get on 5G but not 2.4G.
Before you try to get on the 2.4G again, you need to unplug the Dot for 5 minutes or a little longer. Then, plug it back in and instead of following the instructions from scratch (and since you are on 5G you say) go into the Alexa app, Settings and find your Dot. Go into that area and notice it shows "Update Wi-Fi". It should also show which wi-fi signal you are on, if (as you say you are on 5G} it will indicate that. If it says you are on 2.4G then that is what you want so do nothing except Cancel.
If it says you are on 5G and you want to be on 2.4G, then tap the arrow on the right and follow the on screen directions to change connection to 2.4G.