Website not accessible sometimes over 4G router connection

I develop a website which is online since 6 years from now.

It's hosted in a CentOS6 server with Apache, and coded with PHP.

It works perfectly, but one customer said that he loses connection with the website sometimes. From our location, all is OK, and for other customers it's OK too. He is working over a 4G router and other websites like google.com or facebook.com load OK in his PC.

If I access the website over my 4G connection on my smartphone, it's OK too.

The requests that he creates when he loses connection, appear as Stalled in Chrome network tab, and after some random time, connections show the err_connection_timed_out error. So, this requests don't reach the servers, and Apache is not logging them. It happens with standard HTTP requests and XHR requests.

The website is hosted in IONOS. It has a load balancer (created with IONOS cloud) in front of two servers with Apache, and it works over HTTPS.

I suspect that the issue is in the customer network or router, but I can't figure it out.

If anyone has experienced something like this, any help would be awesome. Thanks in advance.