Extremely slow speeds on our mobile phones

[CC27]

We have extremely slow speeds on our mobile phones through WiFi to the extent that sometimes we have to switch WiFi off and connect through cellular. It is becoming unmanageable to the extent that we need to explore alternative providers. It can take a few minutes to load a Safari page on WiFi (very quick on cellular). And the same for the full content of emails to load. We have the same issue at another property where we have Sky Broadband. We don't have the issue in other people's houses. Please advise.