Wi-Fi slow on iPhone, fast on computer or tablet

At our home, we pay for 20 Mbps Internet connection to operator. On computers and tablets we get full speed via Wi-Fi, while on iPhone 7 and iPhone XR we gat max 2.7 Mbps. We live in apartment with maybe 20 different wireless networks. 1. We tried different channels - no change in behaviour on our iPhones. 2. We tried phones at our company Wi-Fi networks - we got around 17 Mbps there, so phone hardware is capable of receiving more data. 3. We bought new modem/router Asus DSL-AC55U which can stream also on 5 Ghz - still same behaviour. 4. We called with provider on the phone trying to change settings - no effect, still same behaviour. 5. we have tried on phones change DNS server to manual, we tried googles and others - still same behaviour. I believe that there is problem with Wi-Fi settings. Anyone has idea how to resolve this issue?