To enable TCP/IP on your old 6710.
1. You must get BES/MDS hosting at $10-25 per month
(But you must already have a BlackBerry plan for this)
2. Or you must upgrade to BlackBerryOS 4 and use the APN for your carrier in Options->TCP->APN.
(No guarantees that this will work.)
Please make sure you are using a 16MB 6710, not an 8MB 6710, though! BlackBerryOS 3.6 is very old, and may not even work with MDS.
I have never heard of anybody getting a network lock due to a BlackBerryOS upgrade, regardless of whether you have a BlackBerry plan or not.
You should be noted that even configuring the APN may not cause Internet software to work. (Even though the APN bypasses the BlackBerry network). You will likely still be charged for data usage even for non-BlackBerry data.