modem or router?
modem is hg 150-ub
router is openwrt pi4
So is the Pi4 handling PPP? If so you should have the logs to prove if PPP is dropping.
If the logs show PPP has dropped multiple times while the AS value from the modem shows it was in sync, you have your answer of what is happening, just not why.
Also the HG150-UB technically appears to be a router itself, is it being used merely as a modem to split the Internet and IPTV onto their own ports? They key here again is where PPP is being handled as you can rarely get some weird connectivity issues if you are in a double-NAT.