As a non-native English speaker, what's a brief, free resource on how to get my accent and fluency up to a "good enough" level similar to how foreign dignitaries speak?

As a non-native English speaker, what's a brief, free resource on how to get my accent and fluency up to a "good enough" level similar to how foreign dignitaries speak? Hi there, I am a 30 year old guy from Bangladesh. My native language is Bengali but I know English as a second language. I studied in an English medium school and graduated with a GCE O and A Levels. Now my problem is, my English writing skills are doable. But when it comes to spoken english: I stutter extremely and my spoken English is downright terrible. My accent is horrible as well. I stutter in Bengali mildly and its not that inappropriate or noticeable. But in English my stuttering is just plain horrible. I get blocks in various words and what I ultimately say comes off to people as something else than what I intended. Other than that, my accent is very thick. I would like to have a standard North American accent. My spoken English is also terrible, with me messing up grammar and style and what not. Its not even 5% near to how native speakers talk. I can't describe it, but its downright bad. I see a lot of non-native English speakers like diplomats and foreign dignitaries who speak in English in international events but do so at a decent level. Their accent, spoken English and fluency are all at a decent or good enough, even though they might not be characterised as "good" and they are certainly nowhere near the level of a native speaker. So for me, I am trying to reach a level of fluency, accent and spoken English where I am at a decent/good enough level like the dignitaries I mentioned above. Therefore can someone suggest a free resource (like a book, online course, videos, apps etc) that will get my speaking skills to a decent level? PS: The book or resource must be a short/brief resource and free. It should not be something that takes a decade to master. I am only looking for good enough improvement and not perfect fluency.