This is a story related achievement so is not missable.
After having successfully had the letter sent to Ryo's father translated by Xiuyu Xia at Russiya China Shop, Ryo will realise that part of the letter is a phone number. You need to find a phone and a phone book, there is one back at the Hazuki Residence in the entranceway, or there is also one at Hirata Tobacco Shop, it's the small shop right at the end of Dobuita, past You Arcade and Smiley Flower Shop (where Yuriko works) near the bus stop and main road. There is also a telephone in Sakuragaoka but there is no phone book there which you will need afterwards.
Once you dial the number you will need to answer the password in the correct order which was also detailed in the letter, you need to answer as follows:
Father's Heaven - Nine Dragons
Mother's Earth - Comrades
After the telephone conversation Ryo will learn that caller is based at a warehouse but doesn't have the location. If you look in the phone book he will learn that the area code is based in Amihama, and after asking around you will further learn that the location is at the harbour and that it can be reached by bus from the end of Dobuita, where you just were if you were at Hirata Tobacco Shop. Wait at the bus stop which is just to the right of the main road after going onto the main road area which is usually inaccessible and then ride the bus to the harbour. Ryo has now reached New Yokosuka Harbour.