I had centralized all my hotspot services, which were using Usermanger as a radius server. It was working perfectly when I had fewer clients, but when the number of clients increased, the Usermanger started misbehaving. Most clients were complaining about their accounts losing profiles or their accounts logging them out before their time limit was reached. So, what is the best solution to this problem?

Use NTP client to make sure that the time is match in all routers

So, what is the best solution to this problem?

Delete all topic with clickbait title.

I have user-manager v6 everywhere and I do not have any of this problem.
Since you do not reveal the configuration of the devices, and the clickbai title,
is clear your are unable to manage correctly the software involved.
Take some courses.

I Solve this by Just installing FreeRadius