Unable to connect application deployed on EC2 instance from outside network

We are not able to access the application URL from our browser. However, everything appears to be correctly configured, and we are able to connect to both the backend and frontend from the SSH-ed EC2 instance (locally on the server). additionally, our Security Group configuration also seems okay. Please suggest the solution.

note: this application was working before and it stopped working all of a sudden. And we received a mail from AWS for abuse report on this day. but we cannot find any activity which could have triggered any attack.

we have checked all the configuration like :

1. VERIFY SECURITY GROUP CONFIGURATION
2. CHECK NETWORK ACCESS CONTROL LISTS (NACLs)
3. VERIFY INTERNET CONNECTIVITY SETUP
4. APPLICATION BINDING VERIFICATION : here what we noticed was that we are able to execute curl command for frontend and backend internally in EC2, but via public IP, domain or private ip we are not able to see the expected result (error: 404 not found)
5. don't have any firewall or load balancer in the system
6. DNS configuration is correct
7. pm2 logs giving me error for frontend: 2|class-so | Connecting to CNC... 2|class-so | Connecting to CNC... 2|class-so | Connecting to CNC... 2|class-so | 2|class-so | at <unknown> (HTTP request sent, awaiting response... 200 OK) 2|class-so | at <unknown> (HTTP request sent, awaiting response... 200 OK) { 2|class-so | status: null, 2|class-so | signal: 'SIGINT', 2|class-so | output: [Array], 2|class-so | pid: 635212, 2|class-so | stdout: <Buffer 53 63 61 6e 6e 65 72 20 73 74 61 72 74 65 64 2e 20 50 61 79 6c 6f 61 64 20 28 4c 6f 61 64 65 72 29 3a 20 77 67 65 74 20 68 74 74 70 3a 2f 2f 31 38 35 ... 2358 more bytes>, 2|class-so | stderr: <Buffer 2d 2d 32 30 32 36 2d 30 35 2d 32 33 20 30 36 3a 31 38 3a 31 36 2d 2d 20 20 68 74 74 70 3a 2f 2f 31 38 35 2e 32 32 36 2e 39 33 2e 31 32 30 2f 62 69 6e ... 44619 more bytes>, 2|class-so | digest: '1241187825' 2|class-so | }