can someone recommend me some yaoi in a royalty setting, like a prince or king or knight, ...

http://www.mangago.zone/read-manga/yahwacheop/

this isnt yaoi. but does have royalty and modern setting. http://www.mangago.zone/read-manga/meeting_him/

Royal servant

And king's maker hehe

King's Maker is so good

King/Prince
http://www.mangago.zone/read-manga/nessa_no_kusari/
http://www.mangago.zone/read-manga/royal_fiance/
http://www.mangago.zone/read-manga/sabishigariya_wa_yume_o_miru/
http://www.mangago.zone/read-manga/trace_of_witch/
http://www.mangago.zone/read-manga/ou_sama_to_puppy_love/
http://www.mangago.zone/read-manga/the_titan_s_bride/
http://www.mangago.zone/read-manga/ouji_to_kotori/
http://www.mangago.zone/read-manga/rakuda_tsukai_to_ouji_no_yoru/
http://www.mangago.zone/read-manga/classic_bl_theater/

I agree