Please excuse me, I realise your question was directed at the Op but I can't resist. Mine was AC/DC Back In Black. Cranked all the way up to 11 because that is the only way to listen to that masterpiece.📢🎤🎸🥁📢

Good luck. Please keep us updated. Thank you. :)

I based my comment purely on the photo that the OP provided which looked to me like the grout was way past the point of any concern for sealants that may or may not have been applied previously. With no disrespect intended to the OP, is is clearly a well aged bathroom. Ask me how I know, lol. 😁

Toilet cleaner gel with bleach - one product. Brand does not matter. Use product over the grout and let sit for 5 minutes. Use old toothbrush and scrub into grout. Use old rag with water to rinse. The rest of the day is yours to enjoy. :)