Your 2.4GHz wireless connection with 20MHz channel bandwidth is restricting your speed.
Technically, it gets you only upto 75Mbps but there's overheads which will lower it significantly to around 50Mbps .

There's no way you're going to reach anything near 300Mbps with 2.4GHz wifi without investing in better hardware which uses 5GHz or at least 2x2 MIMO 2.4GHz at both ends of your router and wifi adapter/receiver.

If despite using wider channels for your 2.4GHz wifi and you're still not connecting close to 100Mbps(max speed after overheads), yet still connecting similarly to 20MHz bands, then your wifi devices might be to be blamed. Either they're too old, incompatible with your router's chipset or do not support wideband 40MHz 2.4GHz standards usually those China unbranded OEM devices suffer from such problems because they don't follow proper standards.

Remember your receiving wireless clients/devices must also match whatever your router throws out. No point buying the best routers out in the market when you use those cheap China OEM tablets to connect.