This has become a regular niggle with SB, particularly after 7pm.
Their internet banking very rarely seems to work at night and the SMS OTP issue when purchasing is becoming worse. I've had 3-4 instances during last week where this happened, mostly after hours, and there is nothing you can do but wait for OTP which is normally received way after the expiry time...
I've also tried changing to e-mail OTP, but for some reason M-web mail is also very much delayed, so the screen usually times out before M-web deliver the mail... thinking of changing my bank profile to my gmail e-mail as these are received within a few seconds.