Is there an option to bond multiple LTE wan (example 3x LHG LTE6) with two Mikrotik via GRE,EoIP or VPN to reduce broadcasting latency?

yes, you can do.
For example RB with few ISP can do a few separate connection to central point and merge them via ipip+bonding or use ospf etc.

But this is advance topic and you should know ROS well.

so ... first ... I am not catholic ( and 'am spending no sympathies for any tribes ... )
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BUT
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Is there an option to bond multiple LTE wan (example 3x LHG LTE6) with two Mikrotik via GRE,EoIP or VPN to reduce broadcasting latency?

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you can avoid congestion-related latency by increasing the bandwidth ... it's a well known strategy to do so ... but for anything else ... you have to ask good mary in czestochowa

Due to quarantine in remote areas where is no fiber, LTE has poor upload, not stable, unsuitable for video conferencing.
It seems that ipip can’t be a bonded, isn’t there a slave list interface after creation? I think eoip only ? Maybe someone tried to bond two lte in broadcast mode? Is there benefit, it worth it?
OSPF uses cost calculation to go over the shortest path first, but in this situation there no priority? all of them are the same cost. Cant predict which provider is better in certain hours.