Plans to build a cellular tower by the North Haverhill Fairgrounds cleared two major hurdles last week. By a 5-0 vote on Tuesday, the Planning Board granted site plan approval for a 166-foot-tall monopole at 12 Tee Pee Lane. And by a 4-0 vote on Thursday, the Zoning Board of Adjustment rescinded its opposition to the project. Those decisions allow Vertex Tower Assets LLC to seek a building permit from the Select Board, the final step before construction can begin. The monopole would address the town’s notorious lack of cell service.