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Your iPhone 8 depending on the version has LTE Cat.12 or Cat.16 module while RUT360 uses LTE Cat.6. In this link, you can find detailed information and tips.
Also, try making a few tests and check the average speeds, they may vary because of the environmental conditions, just be careful, making LTE speed tests consume a lot of data. When doing tests make sure that other applications are not consuming data as well and your both antennas are connected.
I would like to ask if you could try to lock your router to certain bands? Please Navigate WebUI > Network > Mobile page: https://wiki.teltonika-networks.com/view/RUT360_Mobile#SIM_card_settings Set Band selection to Manual and select your preferred bands.
To begin with, IPhone 11 has a far more advanced LTE Cat 19 modem with download speeds of up to 1.6Gbps compared to the LTE Cat 4 in RUT240 capable of download speeds of up to 150Mbps. The differences are certainly deeper including additional features and signal processing techniques, which allow to achieve better performance in identical conditions.
Nonetheless, you can try a couple of things, which might help to improve your situation.
Some improvement can be expected, however, there are several factors impacting mobile connectivity performance. The following site provides more details: https://wiki.teltonika-networks.com/view/LTE_Speed_Discrepancies.
I will give it a try.