When you do strength training, you should definitely include stretching in it, because during strength training the muscles are very much hammered, tension is accumulated in them and if you don't do stretching afterwards, sooner or later you may face problems, or even worse - get injured, right at the training.... so, your trainer said right, that in addition to strength training you need to do stretching, and for that you can turn to this stretching coach
https://flexderek.com/en/stretchingonline/.
