What's your plan applied in NTT DOCOMO? ahamo, 5G or Xi(4G LTE)?
If you want to use NTT DOCOMO SIM Card with ahamo or 5G plan, you need to disable VoLTE carrier check first.
You can dial "*#*#86583#*#*" to do it.
Also, please check your APN settings, as DOCOMO use "spmode.ne.jp" APN.
---...