What tray should I re-start on? I started Invisalign & was on tray 8 out of 10, worn inconsistently for 2wks. before I quit.

My ortho suggested I start wearing them again immediately from the last tray that fits. I haven't seen him yet because of office remodeling & a booked schedule, but I'd like to re-start wearing trays immediately as recommended to get back on track & finish. My question is, is it ok to re-start at tray 7 if I haven't worn ANYTHING for 18 mo? When I try 7 its tight & doesn't fit against my teeth/there's a huge gap, I don't want my teeth to "loosen," AND I lost trays 2-6. Best advice? Thank you!