Maybe it is easier to use onvista?
"NAME_SECURITY":"GROSSBRIT. 20/26
seems to be GB00BL68HJ26
https://www.onvista.de/anleihen/handelsplaetze/GROSSBRIT-20-26-Anleihe-GB00BL68HJ26
https://api.onvista.de/api/v1/instruments/BOND/172983854/eod_history?idNotation=296369819&range=M1&startDate={TODAY:yyyy-MM-dd:-P1M}
$.datetimeLast[*]
$.last[*]
How to handle with onvista is written down in Historische Kurse von onvista nicht mehr lesbar - #78 by chirlu and following posts.