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What device do you have your sim card in @RicerDalemon
Can you try your sim card in another device as a process of elimination.
The Network Checker gives an indication of speeds but you may not always be able to achieve the fastest speeds.
Factors such as weather, trees and the amount of people in an area using the masts can affect speeds.
From PedroC1999 "Phones do not latch onto the strongest signal.They follow the network assigned procedure, hence why it's not uncommon to be connected to a weaker signal than is strictly available "
There is a network troubleshooting thread you could use the template in so the Vodafone Social Media Team's can check your area.
Current Phone >
Samsung Z Fold³ 5G.
Previous Phone >
Samsung Note 20 Ultra 5G - SM9860.
If you are on a an unlimited plan to achieve the fastest speed possible you need to be on an unlimited max plan, other unlimited plans will have a speed cap, the Red plans are not capped.
I would recommend going through the self help tips below, there is a template to complete and copy back the results to give the Social Team something to go on. Network Issues