You got a domain called hogehoge.com with your name .com.
And we set up heroku and your name .com and confirmed that the rails app deployed with the following url is working.

I have created a subdomain https://contents.hogehoge.com/ and I want to put wp there.

Your name.com because you borrowed a server with the RS plan
I installed wp from the control panel there and it is running,
I get "ERR_NAME_NOT_RESOLVED cannot access this site" when accessing url

When connecting to ftp, a subdomain directory was created and wp was included, so
I think it is a DNS setting, what should I do?

The DNS record is as follows (excerpts from the contents part only)

Navi domain settings for your name

contents.hogehoge.com NS 86400 ns-rs1.gmoserver.jp
contents.hogehoge.com NS 86400 ns-rs2.gmoserver.jp

DNS record in the control panel of the rental server that names your name

A contents 600 3-digit, 2-digit, 3-digit, 3-digit number (I'm sorry, I didn't know if I could show it or not.
A www.contents 600 3-digit, 2-digit, 3-digit, 3-digit number
MX contents 600 Reference value: mail17.onamae.ne.jp Priority: 10

Thanks for your confirmation