When you say you can't access your account...are you suggesting you can't use your flip phone with your Koodo line, or that you can't log into self-serve?
If you can't access your Koodo line, are you sure your flip phone is 3G compatible? You say you get excellent reception, does that mean someone with a Koodo phone in your area will have no trouble connecting a call or browsing the web on their phone?
Side note: Most flip phones are 2G only, meaning they won't work on Koodo's 3G/4G/LTE network. If it's a CDMA phone, you may have to call customer service to associate that handset with your line, and even then, CDMA will be phased out soon if it hasn't been phased out already.
If you can't log into self-serve, have you tried the "forgot password" option?