I'm surprised you even got that speed? Most would fall back to near dial-up. IMO, mobile providers do this on purpose to frustrate you into upgrading your 4G data allowance? Even though, they could provide much faster fall back speeds without impacting their networks.
I'm still evaluating Verizon's Visible on my iPhone, which has no set data allowance limits on 4G LTE. Getting 140Mbps down. Though primarily use my iPhone as a backup to another 4G LTE router from another provider, and for when traveling to remote work into to my workstation at the office.