Currently I have a galaxy s9 with an unlimited plan at Tmobile. Anytime I try to update apps in the samsung store or update the galaxy app store it either gives me a wifi error code or just won't update
Have you checked galaxy store settings for automatic updates. Three options are available, never, using Wi-Fi only and using Wi-Fi or mobile data.
There's also an option in system settings, data usage, mobile data only apps. The galaxy store is in the list.