The problem I encountered: Originally, the mobile phone had two cards, one for China Mobile and one for China Telecom. Later, I canceled the Telecom card, but the card in the mobile phone has not been taken out, and the upper right corner shows the status of no signal for the Telecom card. Until recently when I started using my phone as a hotspot, I found the hotspot to be very slow.
Solution: Disable the canceled telecom card. After disabling, the no-signal status in the upper right corner will also disappear.