The pronunciation varies hugely.
OED, which tends towards RP in British English, gives
BRITISH ENGLISH /ədˈvəːtᵻsm(ə)nt/uhd-VUR-tuhss-muhnt and /ədˈvəːtᵻzm(ə)nt/ uhd-VUR-tuhz-muhnt
U.S. ENGLISH /ˈædvərˌtaɪzmənt/ AD-vuhr-tighz-muhnt and /ədˈvərdəzmənt/ uhd-VUR-duhz-muhnt
https://www.wordreference.com/definition/advertisement has nine spoken accents
Random House Learner's Dictionary of American English gives
ad•ver•tise•ment /ˌædvɚˈtaɪzmənt, ædˈvɜrtɪsmənt, -tɪz-/
Random House Unabridged Dictionary of American English gives
ad•ver•tise•ment (ad′vər tīz′mənt, ad vûr′tis mənt, -tiz-),
Collins Concise English Dictionary gives
advertisement, sometimes US advertizement /ədˈvɜːtɪsmənt -tɪz-/
and MW gives ad·ver·tise·ment ˌad-vər-ˈtīz-mənt əd-ˈvər-təz-mənt, -tə-smənt
Given the above, the simplest advice on pronunciation is "Whatever you think it is." I don't think it is useful to be prescriptive or attempt classification.