After the commercial coverage of 5G network is deployed, it will not affect the use of 4G at present. 4G phones can still use 4G networks but not 5G. Moreover, 4G and 5G will coexist for a long time and 4G will not be affected by 5G for the time being.
5G turn on method:
realme UI 3.0 & above versions: Settings > Mobile network > select the SIM card > 5G
realme UI 2.0 & 1.0: Settings > SIM card & mobile data > select the SIM card > 5G