TLR (Tubeless ready, not UST) bontrager tyres I've used before have been fine, namely Mud X's
I'm trying to fit them to Mavic XM719 disc rims (tubed rims) which are usually such an easy fit that you if you don't feed the tyre on with two hands working seperately round, the loose tyre 'chases' your progress round the rim on coming off on the opposite side
I've not had a problem fitting any other tyre (whereas DT rims I find difficult for eg), until these

Is there anything I can use to help them slip/slide on?

try some washing up liquid get bead of tyre into middle of the rim and then work other bead on they tend to be very tight but do go on

If you are using tubeless rimstrips, that can make it difficult to get the tyres on as they prevent the tyre dropping into the middle of the rim.

I bought a pair of these tyres to fit on Mavic rims. One went on OK with a little resistance the other one I couldn't get the last 5 inches of bead on no matter what I did (even tried metal tyre levers) & gave up. It then took me another 45mins to get the bloody thing off the rim – Grrrr.

I gave the second one to a mate & he had the same fun & games trying to get it on & even he gave up.

Anyone want a brand new Bonti ACX TR tyre??

No tubeless rimstrip, I was hoping to use them tubed for now. There is a thick(ish) normal rimstrip sort of stopping it from dropping into the middle though

Right, washing up liquid, I'll give it a try, should I try watered down or neat?