To turn on DNSSEC validation on your recusrsive resolver you only need to enable it and include the root trust anchor. For example on BIND /etc/namedb/named.conf you might look like below. Make sure to remove or comment (//) out "recursion yes" and "dnssec-validation yes" and do a "service named restart" when done with this exercize to return your nameserver to authoritative server mode.
# cat /etc/namedb/named.conf options { directory "/etc/namedb/working"; pid-file "/var/run/named/pid"; dump-file "/var/dump/named_dump.db"; statistics-file "/var/stats/named.stats"; recursion yes; dnssec-validation yes; allow-query { any; }; }; zone "." { type hint; file "/etc/namedb/named.root"; }; trusted-keys { // real root "." 257 3 8 "AwEAAagAIKlVZrpC6Ia7gEzahOR+9W29euxhJhVVLOyQ bSEW0O8gcCjFFVQUTf6v58fLjwBd0YI0EzrAcQqBGCzh /RStIoO8g0NfnfL2MTJRkxoXbfDaUeVPQuYEhg37NZWA JQ9VnMVDxP/VHL496M/QZxkjf5/Efucp2gaDX6RS6CXp oY68LsvPVjR0ZSwzz1apAzvN9dlzEheX7ICJBBtuA6G3 LQpzW5hOA2hzCTMjJPJ8LbqF6dsV6DoBQzgul0sGIcGO Yl7OyQdXfZ57relSQageu+ipAdTTJ25AsRTAoub8ONGc LmqrAmRLKBP1dfwhYB4N7knNnulqQxA+Uk1ihz0="; // class root "." 257 3 8 "AwEAAc3PS/ln9ICSWGS5E9zCZez5sH5EJsqUbbL0V8+R ZsQbpYI1L87gKHVCZgRgCfb1R1Vu+DcbIt6In/B5nvLa IZAe6lcx40Jt+Stm/PvES8YGVcJ9ndeCsp9xadFN5rGN a+pm536xOSm4LizWXvi+ibfsaIGOfUDIYClyHo1xChlG xbomw+tM1AgmKN8GketMGfGQGL03A1Vbeqe8n/cgEp5u C7ifAunAaHzW8R41t29aHyM3LYSyhqhboRbzyeO9FBPO y1r8oz7lJXeA9swBYiUo7p4ExdK+Ln9SUCb7UPnSqXRO /AwmhZr21JiSDzO9ggCYrbMZ37HXguiTBRg5bMU="; };