How can I keep my toliet tissue holder from falling off?

See if you can find a wall stud underneath the plaster and install your holder there. Even if it is only one screw into wood it will be much sturdier.

First you have to fix the crumbling wall. Why is it crumbling? What is the wall made from? How about a toilet paper stand instead, like these:

https://www.bedbathandbeyond.com/store/product/deluxe-pedestal-satin-nickel-toilet-paper-stand/1014192484?skuId=14192484&mcid=PS_googlepla_nonbrand_bath_online&product_id=14192484&adtype=pla&product_channel=online&adpos=1o7&creative=224060487099&device=t&matchtype=&network=g&mrkgadid=3140839786&mrkgcl=609&rkg_id=h-7d53ce6f88a2f77b805fa319710431bb_t-1521817393&gclid=Cj0KCQjwy9LVBRDOARIsAGqoVnveVNUa15xdWTRPHiGrzoJtflGLUPOKCIbfFkQGeasw0YQMX5T4VGkaAoPqEALw_wcB

https://www.potterybarn.com/products/sussex-standing-paper-holder/?catalogId=80&sku=3476822&cm_ven=PLA&cm_cat=Google&cm_pla=Bath%20%3E%20Bath%20Fixtures&kwid=productads-adid^202132197543-device^t-plaid^328117579080-sku^3476822-adType^PLA&gclid=Cj0KCQjwy9LVBRDOARIsAGqoVnsU3bReShbzSeBxIdfePVsAKIAJBdET9M8AdyUVhnBdaBnQva5hMbkaAgasEALw_wcB&cm_ite=[3476822]

https://www.wayfair.com/Better-Living-Products-Free-Standing-Toilet-Paper-Holder-54544-54566-54520-L6143-K~KBW1030.html?refid=GX175121827388-KBW1030_7442644&device=t&ptid=152674399260&network=g&targetid=aud-295866893899:pla-152674399260&channel=GooglePLA&PiID%5B%5D=7442644&gclid=Cj0KCQjwy9LVBRDOARIsAGqoVntq-trPjP7wNOSIGaPlNHgvp0meQnXX_yGOwkZmSmekchIQ-zz9m9IaAo8JEALw_wcB

Haha..I gave up on this and screwed a new holder into the vanity side...worked great !

Hello. You can purchase heavy duty anchors. You would put them in the holes to screw the screws into. You can even find some that will open up once screwed in for better support.

I had the same problem. Rather than continue to battle that I repaired it painted over it and placed a standalone toilet tissue holder in the front corner to where we sit. I like it better because I can stack extra rolls and it looks decorative and we never, well rarely, run out of toilet tissue at the wrong time!