Database to browser, bypass firewall

We have around 1,000 customers. All with a database (SQL Anywhere 16). At the moment we have a few customers that do have a website. To extract data from the database we open a port in the router and forward it to the server. That works fine, but every time the customer changes from internet provider we have to modify the router. Now we like to deploy a website for all of our customers. Every customer shoud be able to connect to his own database. For that amount of server the port opening method is inefficient.

We are looking for a solution to bypass the firewall. We have looked at several methods like a reverse tunnel, websockets and messages queing. But find it hard to find the right solution for our problem.

In short: around 5,000 thousands web clients should be able to connect to their own server (one of the thousand servers), bypassing the firewall.

Who can give me a good starting point?

Tia Hans