Phone's download speed doesn't go over 10 Mbps(1.2 MBps)

I have an ASUS Zenfone Max Pro M1(X00_TD). Lately, I have noticed that my phone's downloading or uploading speeds don't go over 10 to 15 Mbps. It has a 2.4 G wifi and easily used to give me 40-60 Mbps 2-3 months ago. But now I don't know why the speeds never go above 15 Mbps. Initially, I thought my ISP maybe down or the server may not be giving off above 15, but I cross verified it 50 times with other devices. It doesn't go above 15 Mbps even on cellular data. The ISP is fine(100 Mbps). It doesn't go above 15 even in speed tests. How can I fix this? Thanks in advance