[verb (plain)] + とは言うものの [i-adj] + とは言うものの [na-adj] + だ + とは言うものの [noun] + だ + とは言うものの [sentence]。とは言うものの、[sentence]
とは言うものの concedes a point and then qualifies it: "that may be so, but", "having said that". What you concede goes in front in the plain form, and what follows says the situation is not as simple as that. A noun or a な-adjective usually keeps だ.
The と marks the first half as something said or claimed, so it suits a view people hold or a fact that is true only on paper.
とは言うものの can open a new sentence and point back at the one before, like "having said that".
All three concede and then qualify. とは言うものの and とはいえ belong to writing and formal speech. In conversation, say とはいって言っても.