The L3710 hst will have the treadle pedal and three ranges: High, Medium, Low
Creeper gears were an option, but I am not familiar with their operation.
If you have found the range selector and have it in high, maybe make sure the treadle isn't bent? Had a Kioti in the shop with a similar treadle pedal setup and somehow they managed to bend it where it wouldn't have full travel.
I don't think I've ever had a tractor that would reach it's rated top speed on anything except the flat paved road or a slight downhill . Certainly not going up even the slightest slope - even on a hard surface road. An uphill that I can barely detect will keep our tractors from using top gear.
Only tractor I've ever had that had to downshifted going up hill, without a load, was a JD 3020 Propane with a powershift. Either lug it up hill at 1500 rpm in 8th or wound out in 7th.