for those with a Mi Router 4c who have problems with their wifi, you are not alone. It's the same with me when there are many users using wifi, it disappears and I notice that port 53 udp can be exploited to bypass the firewall rules when he blocks port 53 udp, he no longer has any problems. may be the port 53 is the problem because it is open port. this first solution its worked well but the DNS is listening on port udp 53 so if this port is blocked. you cant connect to he internet: UDP 53 Bypass Firewall Rules Vulnevility
2nd Solution:
X-WRT is the Chinese Fork of Openwrt where the support of wifi drivers is better because the Mi Router 4c is also Chinese so maybe I will switch here. I will try but if there are users of mi router 4c who switch to X-WRT you can chat at below if you still have problems.:
4th Solution
I have requested today in the DD-WRT Forum if they can support the Mi Router 4c with the router requirements, there is no problem. but I just don't know how to get cfe.bin & nvram.bin via safe