Huawei B528 needs constant restarts

I have a Huawei B528 for my internet connection (there is no dsl available here), but it is giving me a lot of trouble. I have to restart the router 3 to 4 times a week, because the speed of the internet connection just gets unbearibly slow. There are at most about 3 devices connected to it at a time, so it shouldn't really be overloaded.
After a restart it usually works great for about an hour and then slows down a bit again, but stays usable for up to a day or two, after that it just gets so slow, it takes a minute to do a single google search.

Unfortunately there is no OpenWRT firmware available for this router otherwise I would have installed it a while ago...

I don't think its overheating, as restarting it immediately fixes the issue.
The firmware is up to date.

Is there anything I can do to fix this problem apart from buying a different router?
I don't want to buy a router now, because I am getting fibre in the near future...

Use a timer , reboot it every night.