Some carriers detect video streaming and throttle your internet speed. The streaming service detects the low-speed connection and reduces the resolution to match.
If that's what's happening, you can either use WiFi instead of mobile data, or else try to bypass the carrier throttling by using a VPN. But most commercial VPNs won't support high-speed connections unless you pay a lot, and your carrier might throttle you anyway if it detects an IP of a known VPN provider.. I run my own VPN at home on a Raspberry PI. Connecting my phone to that VPN, I can successfully evade my carrier's throttling, and the VPN is fast enough to support 4k streaming.