all routers/modems has hardware limit on processing, basically faster speed need higher power (modem/router)processor, keep in mind that you can try making 10K connection link at the same times throught that little modem/router. you can manully limit/picked the connectin node in BT software to get a faster but stable download/upload. never trust the brand, big barnd like Dlink, Netgear still have shit processor. u can go for comercial class hardware which have much more capability but double price. wireless connection will need almost double processor power.
when a processor overrun it will freeze or restart. which likely is ur case.
also, the internet speed ur ISP advertised is MAX speed. line connection will have a lower stable speed, different ISP OR area will have different stable speed, u can contact ur ISP to find out.