Re: Please try <painful solution that is worse than bug>
The last Samsung phone I had was an S5, but that had an option to download using both 4G and WiFi connections simultaneously to boost speed. Might be worth checking if this is still a feature and if it's enabled.
Edit: seems that the S9 does have the feature. See https://www.verizonwireless.com/support/knowledge-base-216765/