Products that can remind my mother to go to the bathroom?

It sounds like you and your father are in a bit of denial about your mothers incontinence.
Life will be much easier for all involved if you just let your mother pee and poop in her diapers,
Your mothers brain is broken and it can no longer figure out when to pee/poop and when not too.
It's heartbreaking....I know. My late husband who had vascular dementia ended up having to get a supra pubic catheter placed because his incontinence was off the charts.
It's really not worth getting your mother or your father upset over this very common issue with someone with dementia. Caring for someone with dementia is hard enough without causing undue stress and strife over going to the bathroom.
Often incontinence is the straw that will break the camels back(especially when it's a husband caring for a wife)and will cause them to get their loved one placed.
Perhaps your father is at that point.

You will probably be more successful having her use the toilet at "normal" times throughout the day and directing her to the bathroom when she is otherwise on her feet, most logically before meals to wash up and after meals to freshen up. After breakfast is a good time to try for a bowel movement and to get her washed and dressed for the day. Before/after any change in activity, like before and after walks, or going outside.
It's going to be harder to get her up and going if she tends to sit on the couch all day so keeping her moving periodically is beneficial in more ways than one.

Your father is the best reminder to mom that she needs to use the bathroom at 2 hour intervals. Plus, she needs his help in there making sure she's cleaned up properly, her brief is put back on properly and shes not flushing rolls of tp down the toilet. A timer or other reminders do not work with dementia, only human prompts will work.

Best of luck.

Sometimes it's in the approach...maybe he could say something like "why don't we both go to the bathroom and wash up before we get settled in for breakfast". Or "let's go to the bathroom before we take a nap". I have found that if I ask my mom a direct question "do you want to use the toilet"...she will say no. Do you want to take a shower...no. If I pose it as "this is what we're doing", she seems to go along with it more times than not.

Incontinence is usually the breaking point where most decide to place their family member because the facilities have the staff and equipment to handle it. Toileting and bathing are hard to manage at home. To get involved in these very personal tasks for a mentally incompetent adult is totally different than managing it for a baby.

The best device is an AL facility. She needs to be taken out of the house. She needs more care than your dad can provide.

I've learned that inability to understand words results in accidents. "Toilet" is not a word a person with severe dementia might recognize. Knowing how to use this unnameable object is another thing that they may not know. Even showing it to them doesn't mean they know to sit on it. If "sit" is not a recognizable word, you can get them in the bathroom, point to the toilet, say sit, and even if they do it, they may not produce the desired results. They may have known all of this a week ago, but they don't know it now.

My mom took care of my great-grandmother one summer. I recall that mom had the hardest time getting GG'ma to sit after she got her in the bathroom! As a child I found that quite strange.

Some things never change.

Moral of the story: Don't assume that your dementia person knows the basics anymore. Same thing with "Put your shoes on" if they don't know what a shoe is and where it goes. Same thing with "Take your pills" if they don't recognize those round objects as pills and don't think they've ever taken them before.

Maybe a cooking timer to go off every few hours. But It sounds like this may soon get to be to much for your dad, have you talked about options when your dad can't do it anymore. Caregiving is really hard on the caregiver, so I you chose to keep mom home, don't forget your dad's health, make sure he is taking care of himself, eating right going to docs, and getting breaks.

Best of luck to you

It is up to dad to say to mom..."honey, time to go to the bathroom" and this is not a question but a statement that he then needs to follow through with.
Every 2 hours.
He can connect it with something she likes.
First bathroom break in the morning she can get some juice or tea...
Second break a bit later some fruit.
Third Lunch time
Fourth maybe a walk around the neighborhood
Fifth a bit of fruit.
Another bathroom break after dinner and before bed. last check and change at bedtime.
It sounds like a lot but it keeps her moving and that helps blood flow and that helps prevent pressure sores

Her brain won’t understand what those reminders mean.

I agree that your Mom is too far along in her dementia journey to be able to learn new things, no matter how simple. She's not even retaining the things that are currently in her memory.

The strategies that others have posted are really good ones. Your Dad is now the only person who can change, not your Mom. He needs to come to grips with this.