If the underboob band mark area (or any skin-fold or skin-to-skin contact area) is itchy and red, it is likely to have a fungal infection. If you shower in the evening, air-dry, and sleep bra-less so that nothing irritates the under-boob, but find that you still have a red mark in the morning - even if it is not itchy - then you very likely still have a fungal infection.
IME the only solution is to treat the infection. Otherwise, no matter what you do, it keeps coming back.
Shower twice a day with gentle soap or shower gel on a washcloth. If you you cannot shower twice a day, then at least clean the affected area that way. Towel dry. (I use a fresh facecloth every day to dry that area while the infection is active, but I don’t know if that is strictly necessary.)
Apply Canesten to clean, dry skin twice a day.
Once the redness is gone, continue applying Canesten in the morning, but apply talc in the evening for another week. Then switch to talc morning and evening for as long as you wish.
Wear a fresh bra every day while treating the thrush.
Wear a fresh bra every day that you get sweaty.
Do not put anti-persperant or deodorant on your underboob until the thrush is completely cleared.